docs: document trusted_proxies and record in TODO.md

Describe the trusted_proxies key in README.md and config.example.yml,
including the rightmost-non-proxy selection rule and the safe default of
trusting no one. Record the unit in TODO.md.

Model: opus-4-8

Dockerfile

@@ -67,8 +67,9 @@ RUN adduser -D -H -s /sbin/nologin pixad && \
     mkdir -p /var/lib/pixa /etc/pixa && \
     chown pixad:pixad /var/lib/pixa
 
-# Copy default config (edit signing_key before use)
+# Copy the image config; signing_key comes from PIXA_SIGNING_KEY.
-COPY config.example.yml /etc/pixa/config.yml
+# Mount a file over /etc/pixa/config.yml to override anything else.
+COPY config.docker.yml /etc/pixa/config.yml
 
 USER pixad
 WORKDIR /var/lib/pixa

README.md

@@ -15,14 +15,25 @@ git clone https://git.eeqj.de/sneak/pixa.git
 cd pixa
 make build
 
-# run with a config file
+# run with a config file: copy the example and set a real signing key
-./bin/pixad --config config.example.yml
+# (the example placeholder is refused at startup), e.g. with
+# openssl rand -base64 32
+cp config.example.yml config.yml
+$EDITOR config.yml   # replace the signing_key placeholder
+./bin/pixad --config config.yml
 
 # or build and run via Docker
 make docker
-docker run -p 8080:8080 pixad:latest
+docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
 ```
 
+A container is configured two ways. The signing key comes from the
+`PIXA_SIGNING_KEY` environment variable, which the baked-in config
+reads; if it is unset the container exits at startup naming the
+variable. Everything else uses built-in defaults, so to change any
+other setting mount your own file over `/etc/pixa/config.yml` (see
+`config.example.yml` for the full set of keys).
+
 ## Rationale
 
 Image-heavy web applications need a fast, caching reverse proxy that
@@ -75,7 +86,7 @@ or partial matching is supported.
 **Signed data format** (colon-separated):
 
 ```
-HMAC-SHA256(secret, "host:path:query:width:height:format:expiration:quality:fit")
+HMAC-SHA256(secret, "host:path:query:width:height:format:expiration")
 ```
 
 Where:
@@ -87,20 +98,13 @@ Where:
 - `height` — requested height in pixels, `0` for original
 - `format` — output format (jpeg, png, webp, avif, gif, orig)
 - `expiration` — Unix timestamp when signature expires
-- `quality` — output quality 1-100; sign `85` (the default) when the URL
-  omits the `q` parameter
-- `fit` — fit mode (cover, contain, fill, inside, outside); sign `cover`
-  (the default) when the URL omits the `fit` parameter
 
-The `q` and `fit` query parameters are covered by the signature. A URL
+**Example:** resize
-signed for one quality or fit value will not verify when replayed with a
+`https://cdn.example.com/photos/cat.jpg` to 800x600 WebP with
-different value; the effective (post-default) value is what is signed.
+expiration 1704067200:
-
-**Example:** resize `https://cdn.example.com/photos/cat.jpg` to 800x600
-WebP with expiration 1704067200, default quality and fit:
 
 1. Build input:
-   `cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200:85:cover`
+   `cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200`
 2. Compute HMAC-SHA256 with your secret key
 3. Base64URL-encode the result
 4. URL:
@@ -118,6 +122,19 @@ Configured via YAML file (`--config`). Key settings:
 
 - `access_control_allow_origin` — CORS origin
 - `allowlist_hosts` — list of allowed upstream hosts
+- `blocked_networks` — list of CIDR ranges to refuse for SSRF protection,
+  added to the always-enforced built-in ranges (loopback, private,
+  link-local, CGNAT, benchmark, NAT64, and the like); an invalid CIDR
+  aborts startup
+- `trusted_proxies` — list of CIDR ranges of the reverse proxies in front
+  of pixa. `X-Forwarded-For` is believed only when the direct peer falls
+  inside one of these ranges; the logged and login-recorded client
+  address is then the rightmost forwarded entry that is not itself a
+  trusted proxy. Otherwise the direct peer address is used and the header
+  is ignored, so a client connecting directly cannot spoof its address.
+  Omitted or empty trusts no one; an invalid CIDR aborts startup. Set
+  this to your proxy's address range when deploying behind a reverse
+  proxy
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout

TODO.md

@@ -1,28 +1,63 @@
 # Workflow
 
-* branch (from `main`)
+* branch per issue from `next`
 * do the work in Next Step
 * move Next Step to the top of Completed Steps
 * move the top item of Future Steps into Next Step
 * commit (`TODO.md` changes in the same commit as the work)
-* merge to `main` if the branch is not protected, otherwise open a PR
+* open a PR based on `next`
+* an independent reviewer who did not write the change gates it
+* the manager squash-merges the PR into `next` once review passes
+* `next` stays green and mergeable to `main` at any time; only the owner
+  merges `next` into `main`, via the single milestone PR
 * push
 
 # Status
 
-pre-1.0. No git tags exist. Recent work extracted the internal/magic,
+pre-1.0. No git tags exist. The `1.0.0` milestone is in progress; work
+lands on `next`, and `main` receives only the milestone PR that the
+owner merges. `next` is at the canonical `golangci-lint` v2.12.2 config
+and is green. Recent work extracted the internal/magic,
 internal/allowlist, internal/httpfetcher, and internal/signature
-packages. The gosec findings from the 2026-07-06 survey are resolved
+packages. The gosec findings from the 2026-07-06 survey are resolved.
-and `make check` is green on main. The disk cache is now size-bounded
+The disk cache is now size-bounded with LRU eviction
-with LRU eviction (`cache_max_bytes`), closing the unbounded disk
+(`cache_max_bytes`), closing the unbounded disk growth DoS vector.
-growth DoS vector.
 
 # Next Step
 
-P1: implement blocked networks configuration to extend SSRF protection
+P1: rate limit global concurrent upstream fetches to prevent resource
+exhaustion
 
 # Completed Steps
 
+- 2026-09-21 trusted-proxy client IP resolution (closes #94): a
+  `trusted_proxies` config key taking a list of CIDRs, parsed by the same
+  `net/netip` list parser as `blocked_networks` (an invalid entry aborts
+  startup naming the key and value; omitted or empty trusts no one); a new
+  `internal/clientip` package resolves the client address by honoring
+  `X-Forwarded-For` only when the direct peer is a trusted proxy, walking
+  the chain right-to-left to the rightmost non-proxy entry, so a client
+  connecting directly cannot spoof its address; the resolved address is
+  stored in the request context by a new middleware and used by the
+  request-logging middleware and the login-attempt logs in place of the
+  raw peer address; documented in `README.md` and `config.example.yml`.
+- 2026-09-21 blocked networks configuration extending SSRF protection: a
+  `blocked_networks` config key taking a list of CIDRs (parsed with
+  `net/netip`, an invalid entry aborts startup naming the key and value),
+  added to the built-in blocklist rather than replacing it; the built-in
+  ranges extended to CGNAT `100.64.0.0/10`, IETF protocol assignments
+  `192.0.0.0/24`, benchmark `198.18.0.0/15`, and NAT64 `64:ff9b::/96`
+  (IPv4-mapped forms covered); enforcement stays in the dial-time
+  re-resolution so the DNS-rebinding window remains closed; documented in
+  `README.md` and `config.example.yml`.
+- 2026-09-21 http.Server hardening (closes #92): added
+  `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
+  `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the
+  existing timeouts and wired them onto the server; added a `LimitBody`
+  middleware capping the two form POST bodies (`POST /`, `POST /generate`)
+  at `MaxFormBytes` (1 MiB) and returning 413, applied ahead of the CSRF
+  middleware so an oversized body is refused as 413 rather than being read
+  as a missing CSRF token (403); left `WriteTimeout` at 60s unchanged
 - 2026-08-07 update golangci-lint to v2.12.2 with the canonical
   `.golangci.yml` (v2 schema, `default: all` minus six disabled
   linters, `lll` 88, tests included): bumped the pinned
@@ -116,8 +151,6 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Future Steps
 
-- P1: rate limit global concurrent upstream fetches to prevent
-  resource exhaustion
 - P1: strip EXIF and other metadata from processed images (privacy)
 - P2: security
     - referer blacklist

config.docker.yml

@@ -0,0 +1,11 @@
+# Pixa configuration baked into the Docker image.
+#
+# The signing key is read from the PIXA_SIGNING_KEY environment
+# variable; startup aborts naming it when it is unset. Every other key
+# is omitted so its default applies. Operators who need more (an
+# allowlist, metrics, and so on) mount their own file over
+# /etc/pixa/config.yml.
+
+signing_key: "${ENV:PIXA_SIGNING_KEY}"
+state_dir: /var/lib/pixa
+port: 8080

config.example.yml

@@ -22,6 +22,27 @@ allowlist_hosts:
   - github.com
   - user-images.githubusercontent.com
 
+# Additional CIDR ranges to refuse when fetching upstream, extending the
+# SSRF protection. These are added to the always-enforced built-in ranges
+# (loopback, RFC 1918 private, link-local, CGNAT, benchmark, NAT64, and
+# similar), never replacing them. Each entry must be a valid CIDR in IPv4
+# or IPv6 form; an invalid entry aborts startup.
+# blocked_networks:
+#   - 100.64.0.0/10
+#   - 2001:db8::/32
+
+# CIDR ranges of the reverse proxies in front of pixa. X-Forwarded-For
+# is believed only when the direct peer is inside one of these ranges;
+# the client address in the access log and login records is then the
+# rightmost forwarded entry that is not itself a trusted proxy. A client
+# connecting directly (peer outside these ranges) cannot spoof its
+# address: the header is ignored and the peer address is used. Omitted or
+# empty trusts no one; an invalid CIDR aborts startup. Set this when
+# deploying behind a proxy.
+# trusted_proxies:
+#   - 10.0.0.0/8
+#   - 2001:db8::/32
+
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
 allow_http: false
 

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/getsentry/sentry-go v0.40.0
 	github.com/go-chi/chi/v5 v5.2.3
 	github.com/go-chi/cors v1.2.2
+	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/securecookie v1.1.2
 	github.com/prometheus/client_golang v1.23.2
 	github.com/slok/go-http-metrics v0.13.0

go.sum

@@ -175,6 +175,8 @@ github.com/googleapis/enterprise-certificate-proxy v0.3.6 h1:GW/XbdyBFQ8Qe+YAmFU
 github.com/googleapis/enterprise-certificate-proxy v0.3.6/go.mod h1:MkHOF77EYAE7qfSuSS9PU6g4Nt4e11cnsDUowfwewLA=
 github.com/googleapis/gax-go/v2 v2.14.2 h1:eBLnkZ9635krYIPD+ag1USrOAI0Nr0QYF3+/3GqO0k0=
 github.com/googleapis/gax-go/v2 v2.14.2/go.mod h1:ON64QhlJkhVtSqp4v1uaK92VyZ2gmvDQsweuyLV+8+w=
+github.com/gorilla/csrf v1.7.3 h1:BHWt6FTLZAb2HtWT5KDBf6qgpZzvtbp9QWDRKZMXJC0=
+github.com/gorilla/csrf v1.7.3/go.mod h1:F1Fj3KG23WYHE6gozCmBAezKookxbIvUJT+121wTuLk=
 github.com/gorilla/securecookie v1.1.2 h1:YCIWL56dvtr73r6715mJs5ZvhtnY73hBvEF8kXD8ePA=
 github.com/gorilla/securecookie v1.1.2/go.mod h1:NfCASbcHqRSY+3a8tlWJwsQap2VX5pwzwo4h3eOamfo=
 github.com/grpc-ecosystem/grpc-gateway/v2 v2.26.3 h1:5ZPtiqj0JL5oKWmcsq4VMaAW5ukBEgSGXEN89zeH1Jo=

internal/clientip/clientip.go

@@ -0,0 +1,119 @@
+// Package clientip resolves the real client IP address of an HTTP request
+// when pixa runs behind a reverse proxy. Forwarding headers are believed
+// only when the immediate peer is a configured trusted proxy, so an
+// untrusted client cannot spoof its address by sending the header.
+package clientip
+
+import (
+	"context"
+	"net"
+	"net/netip"
+	"slices"
+	"strings"
+)
+
+// ForwardedForHeader is the request header carrying the proxy chain. It is
+// honored only when the immediate peer is a trusted proxy.
+const ForwardedForHeader = "X-Forwarded-For"
+
+// Resolver determines the client IP of a request against a fixed set of
+// trusted proxy networks.
+type Resolver struct {
+	trusted []netip.Prefix
+}
+
+// NewResolver returns a Resolver that trusts forwarding headers only from
+// peers inside the given CIDR ranges. A nil or empty list trusts no one,
+// so the peer address is always used.
+func NewResolver(trusted []netip.Prefix) *Resolver {
+	return &Resolver{trusted: trusted}
+}
+
+// Resolve returns the client IP for a request whose direct peer is
+// remoteAddr (a "host:port" string as in http.Request.RemoteAddr) and
+// whose X-Forwarded-For header lines are forwardedFor (as returned by
+// http.Header.Values). When the peer is not a trusted proxy, the peer
+// address is returned and the header is ignored entirely. When the peer is
+// trusted, the header is walked right to left and the first address that is
+// not itself a trusted proxy is returned; this is the client the outermost
+// trusted proxy observed, and entries an untrusted client may have prepended
+// sit to its left and are never reached.
+func (r *Resolver) Resolve(remoteAddr string, forwardedFor []string) string {
+	peer := hostOnly(remoteAddr)
+
+	peerAddr, err := netip.ParseAddr(peer)
+	if err != nil || !r.isTrusted(peerAddr) {
+		return peer
+	}
+
+	for _, hop := range slices.Backward(forwardedForChain(forwardedFor)) {
+		hopAddr, err := netip.ParseAddr(hop)
+		if err != nil || r.isTrusted(hopAddr) {
+			continue
+		}
+
+		return hopAddr.String()
+	}
+
+	return peerAddr.String()
+}
+
+// isTrusted reports whether addr falls inside one of the trusted proxy
+// ranges. Addresses are unmapped first so an IPv4-mapped IPv6 form matches
+// an IPv4 range, matching the fetcher's blocklist comparison.
+func (r *Resolver) isTrusted(addr netip.Addr) bool {
+	if !addr.IsValid() {
+		return false
+	}
+
+	unmapped := addr.Unmap()
+
+	return slices.ContainsFunc(r.trusted, func(prefix netip.Prefix) bool {
+		return prefix.Contains(unmapped)
+	})
+}
+
+// hostOnly strips the port from a "host:port" address. A value without a
+// port (already a bare host) is returned unchanged.
+func hostOnly(remoteAddr string) string {
+	host, _, err := net.SplitHostPort(remoteAddr)
+	if err != nil {
+		return remoteAddr
+	}
+
+	return host
+}
+
+// forwardedForChain flattens the comma-separated entries of every
+// X-Forwarded-For header line into a single ordered, trimmed list.
+func forwardedForChain(values []string) []string {
+	var chain []string
+
+	for _, value := range values {
+		for part := range strings.SplitSeq(value, ",") {
+			trimmed := strings.TrimSpace(part)
+			if trimmed != "" {
+				chain = append(chain, trimmed)
+			}
+		}
+	}
+
+	return chain
+}
+
+// contextKey is the private key type under which the resolved client IP is
+// stored in a request context.
+type contextKey struct{}
+
+// WithClientIP returns a copy of ctx carrying the resolved client IP.
+func WithClientIP(ctx context.Context, ip string) context.Context {
+	return context.WithValue(ctx, contextKey{}, ip)
+}
+
+// FromContext returns the resolved client IP stored in ctx, or an empty
+// string if none was set.
+func FromContext(ctx context.Context) string {
+	ip, _ := ctx.Value(contextKey{}).(string)
+
+	return ip
+}

internal/clientip/clientip_test.go

@@ -0,0 +1,189 @@
+package clientip_test
+
+import (
+	"net/netip"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/clientip"
+)
+
+// Addresses reused across the resolver cases.
+const (
+	trustedRangeV4 = "10.0.0.0/8"
+	forwardedV4    = "203.0.113.7"
+	untrustedV4    = "198.51.100.9"
+	trustedPeer    = "10.0.0.1:5000"
+)
+
+// mustPrefixes parses CIDR strings into prefixes for building a resolver.
+func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
+	t.Helper()
+
+	prefixes := make([]netip.Prefix, 0, len(cidrs))
+
+	for _, c := range cidrs {
+		p, err := netip.ParsePrefix(c)
+		if err != nil {
+			t.Fatalf("netip.ParsePrefix(%q) error = %v", c, err)
+		}
+
+		prefixes = append(prefixes, p)
+	}
+
+	return prefixes
+}
+
+type resolveCase struct {
+	name         string
+	trusted      []string
+	remoteAddr   string
+	forwardedFor []string
+	want         string
+}
+
+// runResolveCases runs each case against a resolver built from its trusted
+// list and checks the resolved address.
+func runResolveCases(t *testing.T, cases []resolveCase) {
+	t.Helper()
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			r := clientip.NewResolver(mustPrefixes(t, tt.trusted...))
+
+			got := r.Resolve(tt.remoteAddr, tt.forwardedFor)
+			if got != tt.want {
+				t.Errorf("Resolve(%q, %v) = %q, want %q",
+					tt.remoteAddr, tt.forwardedFor, got, tt.want)
+			}
+		})
+	}
+}
+
+// TestResolvePeerTrust covers the trust decision on the direct peer: a
+// forwarded header is believed only from a trusted peer, and a client
+// connecting directly cannot spoof its address.
+func TestResolvePeerTrust(t *testing.T) {
+	t.Parallel()
+
+	runResolveCases(t, []resolveCase{
+		{
+			name:         "trusted peer honors forwarded client",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{forwardedV4},
+			want:         forwardedV4,
+		},
+		{
+			name:         "untrusted peer ignores forwarded header",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   untrustedV4 + ":33333",
+			forwardedFor: []string{forwardedV4},
+			want:         untrustedV4,
+		},
+		{
+			name:         "spoofed chain from untrusted peer cannot influence result",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   untrustedV4 + ":33333",
+			forwardedFor: []string{"1.2.3.4, 10.9.9.9, 127.0.0.1"},
+			want:         untrustedV4,
+		},
+		{
+			name:         "empty trusted list always uses peer",
+			trusted:      nil,
+			remoteAddr:   forwardedV4 + ":80",
+			forwardedFor: []string{"10.0.0.5"},
+			want:         forwardedV4,
+		},
+		{
+			name:         "trusted peer with no forwarded header uses peer",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: nil,
+			want:         "10.0.0.1",
+		},
+		{
+			name:         "unparseable peer is returned unchanged",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   "garbage",
+			forwardedFor: []string{forwardedV4},
+			want:         "garbage",
+		},
+	})
+}
+
+// TestResolveChainWalk covers walking the X-Forwarded-For chain from a
+// trusted peer to the rightmost entry that is not itself a trusted proxy.
+func TestResolveChainWalk(t *testing.T) {
+	t.Parallel()
+
+	runResolveCases(t, []resolveCase{
+		{
+			name:         "rightmost untrusted entry across a mixed chain",
+			trusted:      []string{trustedRangeV4, "192.168.0.0/16"},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{forwardedV4 + ", 192.168.1.1, 10.0.0.2"},
+			want:         forwardedV4,
+		},
+		{
+			name:         "spoofed client behind a trusted proxy is not believed",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{"1.2.3.4, " + untrustedV4},
+			want:         untrustedV4,
+		},
+		{
+			name:         "chain split across multiple header lines",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{forwardedV4, "10.0.0.2"},
+			want:         forwardedV4,
+		},
+		{
+			name:         "garbage entries are skipped",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{forwardedV4 + ", not-an-ip"},
+			want:         forwardedV4,
+		},
+		{
+			name:         "all-trusted chain falls back to peer",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   trustedPeer,
+			forwardedFor: []string{"10.0.0.9, 10.0.0.2"},
+			want:         "10.0.0.1",
+		},
+		{
+			name:         "trusted IPv6 peer honors forwarded client",
+			trusted:      []string{"2001:db8::/32"},
+			remoteAddr:   "[2001:db8::1]:9000",
+			forwardedFor: []string{forwardedV4},
+			want:         forwardedV4,
+		},
+		{
+			name:         "IPv4-mapped peer matches IPv4 trusted range",
+			trusted:      []string{trustedRangeV4},
+			remoteAddr:   "[::ffff:10.0.0.1]:5000",
+			forwardedFor: []string{forwardedV4},
+			want:         forwardedV4,
+		},
+	})
+}
+
+func TestContextRoundTrip(t *testing.T) {
+	t.Parallel()
+
+	ctx := clientip.WithClientIP(t.Context(), forwardedV4)
+	if got := clientip.FromContext(ctx); got != forwardedV4 {
+		t.Errorf("FromContext = %q, want %q", got, forwardedV4)
+	}
+}
+
+func TestFromContextAbsent(t *testing.T) {
+	t.Parallel()
+
+	if got := clientip.FromContext(t.Context()); got != "" {
+		t.Errorf("FromContext with no value = %q, want empty", got)
+	}
+}

internal/config/blocked_networks_internal_test.go

@@ -0,0 +1,90 @@
+package config
+
+import (
+	"testing"
+)
+
+// TestBlockedNetworksParsed loads a valid blocked_networks list and checks
+// each CIDR is parsed into the resolved prefixes in order.
+func TestBlockedNetworksParsed(t *testing.T) {
+	t.Parallel()
+
+	yamlContent := signingKeyLine + `blocked_networks:
+  - 203.0.113.0/24
+  - 2001:db8::/32
+`
+
+	c, err := configFromYAML(t, yamlContent)
+	if err != nil {
+		t.Fatalf("valid blocked_networks should load, got error: %v", err)
+	}
+
+	want := []string{"203.0.113.0/24", "2001:db8::/32"}
+	if len(c.BlockedNetworks) != len(want) {
+		t.Fatalf("BlockedNetworks = %v, want %d entries", c.BlockedNetworks, len(want))
+	}
+
+	for i, w := range want {
+		if got := c.BlockedNetworks[i].String(); got != w {
+			t.Errorf("BlockedNetworks[%d] = %q, want %q", i, got, w)
+		}
+	}
+}
+
+// TestBlockedNetworksOmittedIsEmpty confirms an omitted key leaves the
+// operator list empty; the built-in defaults still apply in the fetcher.
+func TestBlockedNetworksOmittedIsEmpty(t *testing.T) {
+	t.Parallel()
+
+	c, err := configFromYAML(t, signingKeyLine)
+	if err != nil {
+		t.Fatalf("minimal config should be valid, got error: %v", err)
+	}
+
+	if len(c.BlockedNetworks) != 0 {
+		t.Errorf("BlockedNetworks = %v, want empty", c.BlockedNetworks)
+	}
+}
+
+// TestBlockedNetworksInvalidAbortsStartup checks that malformed values abort
+// startup with an error naming the key and the offending value.
+func TestBlockedNetworksInvalidAbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, []abortCase{
+		{
+			name: "not-a-cidr",
+			yaml: signingKeyLine + `blocked_networks:
+  - not-a-cidr
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, "not-a-cidr"},
+		},
+		{
+			name: "bare-address-without-prefix",
+			yaml: signingKeyLine + `blocked_networks:
+  - 10.0.0.1
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, "10.0.0.1"},
+		},
+		{
+			name: "empty-entry",
+			yaml: signingKeyLine + `blocked_networks:
+  - ""
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks},
+		},
+		{
+			name: "non-string-entry",
+			yaml: signingKeyLine + `blocked_networks:
+  - 42
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks},
+		},
+		{
+			name: "null-value",
+			yaml: signingKeyLine + `blocked_networks:
+`,
+			wantErrSubstrings: []string{keyBlockedNetworks, nullValueText},
+		},
+	})
+}

internal/config/config.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"math"
+	"net/netip"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -42,8 +43,16 @@ const (
 	keyAllowHTTP                  = "allow_http"
 	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
 	keyCacheMaxBytes              = "cache_max_bytes"
+	keyBlockedNetworks            = "blocked_networks"
+	keyTrustedProxies             = "trusted_proxies"
 )
 
+// placeholderSigningKey is the dummy signing_key shipped in
+// config.example.yml. It is 45 characters, so it passes the length
+// check, but it is public in this repository and must be rejected at
+// startup so no deployment ever signs URLs with it.
+const placeholderSigningKey = "CHANGE_ME_generate_with_openssl_rand_base64_32"
+
 // Static validation errors. Each use site attaches the offending key
 // and value by wrapping these with fmt.Errorf and %w.
 var (
@@ -54,6 +63,7 @@ var (
 	errNotAnInteger      = errors.New("not an integer")
 	errNotABoolean       = errors.New("not a boolean")
 	errNotAStringList    = errors.New("not a list of strings")
+	errNotAValidCIDR     = errors.New("not a valid CIDR network")
 	errNotAMetricsMap    = errors.New("not a map of metrics settings")
 	errEmptyListEntry    = errors.New("list contains an empty entry")
 	errEmptyEntry        = errors.New("contains an empty entry")
@@ -61,6 +71,9 @@ var (
 	errPortOutOfRange    = errors.New("outside the valid port range")
 	errTooFewConnections = errors.New("must be at least 1")
 	errValueTooShort     = errors.New("value too short")
+	errPlaceholderKey    = errors.New(
+		"is the placeholder from config.example.yml; " +
+			"generate a real key with: openssl rand -base64 32")
 	errMustBeSetTogether = errors.New("must be set together")
 	errMustNotBeNegative = errors.New("must not be negative")
 	errOverflowsInt64    = errors.New("overflows a 64-bit integer")
@@ -100,6 +113,19 @@ type Config struct {
 	AllowHTTP                  bool     // Allow non-TLS upstream (testing only)
 	UpstreamConnectionsPerHost int      // Max concurrent connections per upstream host
 
+	// BlockedNetworks are operator-supplied CIDR ranges to refuse in
+	// addition to the built-in SSRF blocklist. Enforced by the upstream
+	// fetcher's dialer; the built-in ranges always apply.
+	BlockedNetworks []netip.Prefix
+
+	// TrustedProxies are the CIDR ranges of reverse proxies whose
+	// forwarding headers may be believed. Forwarded headers are honored
+	// only when the immediate peer falls inside one of these ranges;
+	// otherwise the peer address is used and the headers are ignored, so
+	// an untrusted client cannot spoof its address. Empty means trust
+	// nothing and always use the peer address.
+	TrustedProxies []netip.Prefix
+
 	// CacheMaxBytes is the disk cache size limit in bytes. Zero
 	// disables the disk cache entirely. When cache_max_bytes is
 	// omitted from the configuration, this holds the computed default
@@ -168,6 +194,16 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		}
 	}
 
+	blockedNetworks, err := parseCIDRList(sc, keyBlockedNetworks)
+	if err != nil {
+		return nil, err
+	}
+
+	trustedProxies, err := parseCIDRList(sc, keyTrustedProxies)
+	if err != nil {
+		return nil, err
+	}
+
 	loader := &strictLoader{sc: sc}
 
 	c := &Config{
@@ -184,6 +220,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		UpstreamConnectionsPerHost: loader.intVal(
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
 		CacheMaxBytes:   loader.int64Val(keyCacheMaxBytes, 0),
+		BlockedNetworks: blockedNetworks,
+		TrustedProxies:  trustedProxies,
 	}
 
 	// The computed default for cache_max_bytes needs a validated
@@ -215,7 +253,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		return nil, loader.err
 	}
 
-	err := c.validate()
+	err = c.validate()
 	if err != nil {
 		return nil, err
 	}
@@ -299,7 +337,8 @@ func isKnownConfigKey(key string) bool {
 	switch key {
 	case keyDebug, keyMaintenanceMode, keyPort, keyStateDir, keySentryDSN,
 		keyDBURL, keyMetrics, keySigningKey, keyAllowlistHosts, keyAllowHTTP,
-		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, "env":
+		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, keyBlockedNetworks,
+		keyTrustedProxies, "env":
 		return true
 	}
 
@@ -341,10 +380,10 @@ func (c *Config) ensureStateDirWritable() error {
 	return nil
 }
 
-// validate checks that all required configuration values are set and
+// validateSigningKey checks that the signing key is present, long
-// that every value is within its valid range.
+// enough, and not the public placeholder from config.example.yml. The
-func (c *Config) validate() error {
+// key value itself is never echoed in error messages.
-	// The signing key value is never echoed in error messages.
+func (c *Config) validateSigningKey() error {
 	if c.SigningKey == "" {
 		return fmt.Errorf("config key %q: %w", keySigningKey, errValueRequired)
 	}
@@ -356,6 +395,21 @@ func (c *Config) validate() error {
 			keySigningKey, errValueTooShort, minKeyLength, len(c.SigningKey))
 	}
 
+	if c.SigningKey == placeholderSigningKey {
+		return fmt.Errorf("config key %q: %w", keySigningKey, errPlaceholderKey)
+	}
+
+	return nil
+}
+
+// validate checks that all required configuration values are set and
+// that every value is within its valid range.
+func (c *Config) validate() error {
+	err := c.validateSigningKey()
+	if err != nil {
+		return err
+	}
+
 	const maxPort = 65535
 	if c.Port < 1 || c.Port > maxPort {
 		return fmt.Errorf("config key %q: value %d is %w 1-%d",
@@ -778,3 +832,87 @@ func getStringSlice(sc *smartconfig.Config) []string {
 
 	return nil
 }
+
+// parseCIDRList parses the value of the named config key into CIDR
+// prefixes, or returns nil if the key is omitted. It accepts a YAML list
+// of strings or a comma-separated string. An explicitly null value, a
+// wrong type, an empty entry, a non-string entry, or an unparseable CIDR
+// aborts startup naming the key and the offending value; the default
+// (an empty list) applies only to an omitted key.
+func parseCIDRList(sc *smartconfig.Config, key string) ([]netip.Prefix, error) {
+	if sc == nil {
+		return nil, nil
+	}
+
+	raw, ok := sc.Get(key)
+	if !ok {
+		return nil, nil
+	}
+
+	if raw == nil {
+		return nil, errNullConfigValue(key)
+	}
+
+	entries, err := cidrListEntries(raw, key)
+	if err != nil {
+		return nil, err
+	}
+
+	prefixes := make([]netip.Prefix, 0, len(entries))
+
+	for _, entry := range entries {
+		prefix, err := netip.ParsePrefix(entry)
+		if err != nil {
+			return nil, fmt.Errorf("config key %q: value %q is %w",
+				key, entry, errNotAValidCIDR)
+		}
+
+		prefixes = append(prefixes, prefix)
+	}
+
+	return prefixes, nil
+}
+
+// cidrListEntries extracts the raw entries of the named CIDR-list key as
+// trimmed, non-empty strings, from either a YAML list of strings or a
+// comma-separated string. Any other shape is a configuration error.
+func cidrListEntries(raw any, key string) ([]string, error) {
+	switch val := raw.(type) {
+	case []any:
+		entries := make([]string, 0, len(val))
+
+		for _, item := range val {
+			str, ok := item.(string)
+			if !ok {
+				return nil, fmt.Errorf("config key %q: list entry %v (%T) is %w",
+					key, item, item, errNotAString)
+			}
+
+			if strings.TrimSpace(str) == "" {
+				return nil, fmt.Errorf("config key %q: %w",
+					key, errEmptyListEntry)
+			}
+
+			entries = append(entries, strings.TrimSpace(str))
+		}
+
+		return entries, nil
+	case string:
+		entries := make([]string, 0)
+
+		for part := range strings.SplitSeq(val, ",") {
+			trimmed := strings.TrimSpace(part)
+			if trimmed == "" {
+				return nil, fmt.Errorf("config key %q: value %q %w",
+					key, val, errEmptyEntry)
+			}
+
+			entries = append(entries, trimmed)
+		}
+
+		return entries, nil
+	default:
+		return nil, fmt.Errorf("config key %q: value %v (%T) is %w",
+			key, raw, raw, errNotAStringList)
+	}
+}

internal/config/config_validation_internal_test.go

@@ -303,6 +303,11 @@ func invalidHostAndCredentialCases() []abortCase {
 			yaml:              "signing_key: short\n",
 			wantErrSubstrings: []string{keySigningKey},
 		},
+		{
+			name:              "signing_key is the documented placeholder",
+			yaml:              "signing_key: " + placeholderSigningKey + "\n",
+			wantErrSubstrings: []string{keySigningKey},
+		},
 		{
 			name:              "signing_key missing",
 			yaml:              "port: 8080\n",

internal/config/trusted_proxies_internal_test.go

@@ -0,0 +1,65 @@
+package config
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestTrustedProxiesConfig checks the trusted_proxies key wiring: a valid
+// CIDR list lands in TrustedProxies in order, and an omitted key trusts no
+// one. The list parser itself is shared with blocked_networks and is
+// exercised in depth by that key's tests.
+func TestTrustedProxiesConfig(t *testing.T) {
+	t.Parallel()
+
+	t.Run("valid list is parsed in order", func(t *testing.T) {
+		t.Parallel()
+
+		c, err := configFromYAML(t,
+			signingKeyLine+`trusted_proxies: ["10.0.0.0/8", "2001:db8::/32"]`+"\n")
+		if err != nil {
+			t.Fatalf("valid trusted_proxies should load: %v", err)
+		}
+
+		got := make([]string, len(c.TrustedProxies))
+		for i, p := range c.TrustedProxies {
+			got[i] = p.String()
+		}
+
+		if joined := strings.Join(got, ","); joined != "10.0.0.0/8,2001:db8::/32" {
+			t.Errorf("TrustedProxies = %v, want the two ranges in order", got)
+		}
+	})
+
+	t.Run("omitted key trusts no one", func(t *testing.T) {
+		t.Parallel()
+
+		c, err := configFromYAML(t, signingKeyLine)
+		if err != nil {
+			t.Fatalf("minimal config should load: %v", err)
+		}
+
+		if len(c.TrustedProxies) != 0 {
+			t.Errorf("TrustedProxies = %v, want empty", c.TrustedProxies)
+		}
+	})
+}
+
+// TestTrustedProxiesInvalidAbortsStartup checks that an invalid or null
+// value aborts startup with an error naming the key and the offending value.
+func TestTrustedProxiesInvalidAbortsStartup(t *testing.T) {
+	t.Parallel()
+
+	runAbortCases(t, []abortCase{
+		{
+			name:              "invalid cidr",
+			yaml:              signingKeyLine + `trusted_proxies: ["999.0.0.0/8"]` + "\n",
+			wantErrSubstrings: []string{keyTrustedProxies, "999.0.0.0/8"},
+		},
+		{
+			name:              "null value",
+			yaml:              signingKeyLine + "trusted_proxies:\n",
+			wantErrSubstrings: []string{keyTrustedProxies, nullValueText},
+		},
+	})
+}

internal/handlers/auth.go

@@ -2,11 +2,13 @@ package handlers
 
 import (
 	"crypto/subtle"
+	"html/template"
 	"net/http"
 	"net/url"
 	"strconv"
 	"time"
 
+	"sneak.berlin/go/pixa/internal/clientip"
 	"sneak.berlin/go/pixa/internal/encurl"
 	"sneak.berlin/go/pixa/internal/imgcache"
 	"sneak.berlin/go/pixa/internal/templates"
@@ -23,13 +25,13 @@ func (s *Handlers) HandleRoot() http.HandlerFunc {
 
 		// Check if authenticated
 		if s.sessMgr.IsAuthenticated(r) {
-			s.renderGenerator(w, nil)
+			s.renderGenerator(w, r, nil)
 
 			return
 		}
 
 		// Show login page
-		s.renderLogin(w, "")
+		s.renderLogin(w, r, "")
 	}
 }
 
@@ -37,7 +39,7 @@ func (s *Handlers) HandleRoot() http.HandlerFunc {
 func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 	err := r.ParseForm()
 	if err != nil {
-		s.renderLogin(w, "Invalid form data")
+		s.renderLogin(w, r, "Invalid form data")
 
 		return
 	}
@@ -46,8 +48,9 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 
 	// Constant-time comparison to prevent timing attacks
 	if subtle.ConstantTimeCompare([]byte(submittedKey), []byte(s.config.SigningKey)) != 1 {
-		s.log.Warn("failed login attempt", "remote_addr", r.RemoteAddr)
+		s.log.Warn("failed login attempt",
-		s.renderLogin(w, "Invalid signing key")
+			"remote_addr", clientip.FromContext(r.Context()))
+		s.renderLogin(w, r, "Invalid signing key")
 
 		return
 	}
@@ -56,12 +59,13 @@ func (s *Handlers) handleLoginPost(w http.ResponseWriter, r *http.Request) {
 	err = s.sessMgr.CreateSession(w)
 	if err != nil {
 		s.log.Error("failed to create session", "error", err)
-		s.renderLogin(w, "Failed to create session")
+		s.renderLogin(w, r, "Failed to create session")
 
 		return
 	}
 
-	s.log.Info("successful login", "remote_addr", r.RemoteAddr)
+	s.log.Info("successful login",
+		"remote_addr", clientip.FromContext(r.Context()))
 
 	// Redirect to generator page
 	http.Redirect(w, r, "/", http.StatusSeeOther)
@@ -87,7 +91,7 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 
 		err := r.ParseForm()
 		if err != nil {
-			s.renderGenerator(w, &generatorData{Error: "Invalid form data"})
+			s.renderGenerator(w, r, &generatorData{Error: "Invalid form data"})
 
 			return
 		}
@@ -97,7 +101,7 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 		// Validate source URL
 		parsed, err := url.Parse(sourceURL)
 		if err != nil || parsed.Host == "" {
-			s.renderGeneratorWithForm(w, "Invalid source URL", r.Form)
+			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form)
 
 			return
 		}
@@ -108,7 +112,7 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 		token, err := s.encGen.Generate(payload)
 		if err != nil {
 			s.log.Error("failed to generate encrypted URL", "error", err)
-			s.renderGeneratorWithForm(w, "Failed to generate URL", r.Form)
+			s.renderGeneratorWithForm(w, r, "Failed to generate URL", r.Form)
 
 			return
 		}
@@ -121,7 +125,7 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 			expiresAtStr = expiresAt.Format(time.RFC3339)
 		}
 
-		s.renderGenerator(w, &generatorData{
+		s.renderGenerator(w, r, &generatorData{
 			GeneratedURL: generatedURL,
 			ExpiresAt:    expiresAtStr,
 			FormURL:      sourceURL,
@@ -186,15 +190,20 @@ type generatorData struct {
 	FormQuality  string
 	FormFit      string
 	FormTTL      string
+	CSRFField    template.HTML
 }
 
-func (s *Handlers) renderLogin(w http.ResponseWriter, errorMsg string) {
+func (s *Handlers) renderLogin(
+	w http.ResponseWriter, r *http.Request, errorMsg string,
+) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 
 	data := struct {
 		Error     string
+		CSRFField template.HTML
 	}{
 		Error:     errorMsg,
+		CSRFField: csrfField(r),
 	}
 
 	err := templates.Render(w, "login.html", data)
@@ -204,13 +213,17 @@ func (s *Handlers) renderLogin(w http.ResponseWriter, errorMsg string) {
 	}
 }
 
-func (s *Handlers) renderGenerator(w http.ResponseWriter, data *generatorData) {
+func (s *Handlers) renderGenerator(
+	w http.ResponseWriter, r *http.Request, data *generatorData,
+) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 
 	if data == nil {
 		data = &generatorData{}
 	}
 
+	data.CSRFField = csrfField(r)
+
 	err := templates.Render(w, "generator.html", data)
 	if err != nil {
 		s.log.Error("failed to render generator template", "error", err)
@@ -219,9 +232,9 @@ func (s *Handlers) renderGenerator(w http.ResponseWriter, data *generatorData) {
 }
 
 func (s *Handlers) renderGeneratorWithForm(
-	w http.ResponseWriter, errorMsg string, form url.Values,
+	w http.ResponseWriter, r *http.Request, errorMsg string, form url.Values,
 ) {
-	s.renderGenerator(w, &generatorData{
+	s.renderGenerator(w, r, &generatorData{
 		Error:       errorMsg,
 		FormURL:     form.Get("url"),
 		FormWidth:   form.Get("width"),

internal/handlers/auth_clientip_internal_test.go

@@ -0,0 +1,41 @@
+package handlers
+
+import (
+	"bytes"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/clientip"
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// TestFailedLoginLogsResolvedClientIP verifies the failed-login record
+// carries the resolved client IP from the request context, not the raw
+// proxy peer address.
+func TestFailedLoginLogsResolvedClientIP(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	h := &Handlers{
+		log:    slog.New(slog.NewJSONHandler(&buf, nil)),
+		config: &config.Config{SigningKey: testSigningKey},
+	}
+
+	form := url.Values{loginKeyField: {"wrong-key"}}
+	req := httptest.NewRequestWithContext(
+		t.Context(), http.MethodPost, "/",
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req = req.WithContext(clientip.WithClientIP(req.Context(), "203.0.113.7"))
+
+	h.handleLoginPost(httptest.NewRecorder(), req)
+
+	if !strings.Contains(buf.String(), `"remote_addr":"203.0.113.7"`) {
+		t.Errorf("failed-login log missing resolved client IP; got %q", buf.String())
+	}
+}

internal/handlers/auth_csrf_internal_test.go

@@ -0,0 +1,273 @@
+package handlers
+
+import (
+	"context"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"regexp"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// testSigningKey is a throwaway signing key for the CSRF flow tests. It
+// seeds the session manager, the encrypted-URL generator, and the CSRF
+// token key, exactly as the real signing key does in production.
+const testSigningKey = "test-signing-key-0123456789abcdef"
+
+// Form field names used in the CSRF flow tests.
+const (
+	loginKeyField = "key"
+	// gorilla/csrf's default form field name, not a credential.
+	csrfTokenField = "gorilla.csrf.Token" //nolint:gosec // G101 false positive
+)
+
+// csrfFieldPattern extracts the token rendered by csrf.TemplateField into
+// the form. The field name is gorilla/csrf's default.
+var csrfFieldPattern = regexp.MustCompile(
+	`name="gorilla\.csrf\.Token" value="([^"]+)"`)
+
+// newCSRFTestRouter builds a router that mirrors the production wiring for
+// the CSRF-protected UI routes (see server.SetupRoutes): the login and
+// generator forms and their POST targets sit behind the real CSRF
+// middleware. Requests are marked plaintext (Debug: true) so the flow runs
+// over httptest's http transport without an https Referer.
+func newCSRFTestRouter(t *testing.T) (*Handlers, http.Handler) {
+	t.Helper()
+
+	cfg := &config.Config{SigningKey: testSigningKey, Debug: true}
+
+	sessMgr, err := session.NewManager(testSigningKey)
+	if err != nil {
+		t.Fatalf("session.NewManager() error = %v", err)
+	}
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	protect, err := newCSRFProtect(testSigningKey, cfg.Debug)
+	if err != nil {
+		t.Fatalf("newCSRFProtect() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:         slog.New(slog.DiscardHandler),
+		config:      cfg,
+		sessMgr:     sessMgr,
+		encGen:      encGen,
+		csrfProtect: protect,
+	}
+
+	r := chi.NewRouter()
+	r.Group(func(r chi.Router) {
+		r.Use(h.CSRF())
+		r.Get("/", h.HandleRoot())
+		r.Post("/", h.HandleRoot())
+		r.Post("/generate", h.HandleGenerateURL())
+	})
+
+	return h, r
+}
+
+// csrfCredentials performs a GET that renders a form and returns the CSRF
+// cookies the middleware set and the token embedded in the form. Passing
+// the authenticated session cookie renders the generator form instead of
+// the login form.
+func csrfCredentials(
+	t *testing.T, srv http.Handler, reqCookies []*http.Cookie,
+) ([]*http.Cookie, string) {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/", nil)
+	for _, c := range reqCookies {
+		req.AddCookie(c)
+	}
+
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("GET / status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	match := csrfFieldPattern.FindStringSubmatch(rec.Body.String())
+	if match == nil {
+		t.Fatalf("no CSRF token field found in rendered form")
+	}
+
+	return rec.Result().Cookies(), match[1]
+}
+
+// postForm submits form values with the given cookies and returns the
+// recorder.
+func postForm(
+	srv http.Handler, path string,
+	cookies []*http.Cookie, form url.Values,
+) *httptest.ResponseRecorder {
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodPost, path,
+		strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	for _, c := range cookies {
+		req.AddCookie(c)
+	}
+
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// TestLoginPostRejectedWithoutToken verifies that POST / with no CSRF token
+// is rejected. This is login CSRF: no session cookie exists yet, so the
+// protection must rest on a token bound to a pre-session cookie.
+func TestLoginPostRejectedWithoutToken(t *testing.T) {
+	t.Parallel()
+
+	_, srv := newCSRFTestRouter(t)
+
+	rec := postForm(srv, "/", nil, url.Values{loginKeyField: {testSigningKey}})
+
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("POST / without token status = %d, want %d",
+			rec.Code, http.StatusForbidden)
+	}
+}
+
+// TestLoginPostRejectedWithForeignToken verifies that a token that does not
+// match the request's CSRF cookie is rejected: a token minted for one
+// cookie cannot authorize a request carrying a different cookie.
+func TestLoginPostRejectedWithForeignToken(t *testing.T) {
+	t.Parallel()
+
+	_, srv := newCSRFTestRouter(t)
+
+	cookiesA, _ := csrfCredentials(t, srv, nil)
+	_, tokenB := csrfCredentials(t, srv, nil)
+
+	rec := postForm(srv, "/", cookiesA, url.Values{
+		loginKeyField:  {testSigningKey},
+		csrfTokenField: {tokenB},
+	})
+
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("POST / with foreign token status = %d, want %d",
+			rec.Code, http.StatusForbidden)
+	}
+}
+
+// TestLoginPostAcceptedWithValidToken verifies that POST / with a matching
+// cookie and token succeeds: the login is processed and a session is
+// established (303 redirect).
+func TestLoginPostAcceptedWithValidToken(t *testing.T) {
+	t.Parallel()
+
+	_, srv := newCSRFTestRouter(t)
+
+	cookies, token := csrfCredentials(t, srv, nil)
+
+	rec := postForm(srv, "/", cookies, url.Values{
+		loginKeyField:  {testSigningKey},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("POST / with valid token status = %d, want %d",
+			rec.Code, http.StatusSeeOther)
+	}
+
+	var authed bool
+
+	for _, c := range rec.Result().Cookies() {
+		if c.Name == session.CookieName && c.Value != "" {
+			authed = true
+		}
+	}
+
+	if !authed {
+		t.Error("valid login did not set a session cookie")
+	}
+}
+
+// TestGeneratePostRejectedWithoutToken verifies that POST /generate is
+// rejected without a CSRF token even when the request carries a valid
+// authenticated session. The session cookie is not sufficient; the policy
+// requires a CSRF token on this cookie-authenticated form.
+func TestGeneratePostRejectedWithoutToken(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newCSRFTestRouter(t)
+
+	sessionCookie := newSessionCookie(t, h)
+
+	rec := postForm(srv, "/generate",
+		[]*http.Cookie{sessionCookie},
+		url.Values{"url": {"https://example.com/a.jpg"}})
+
+	if rec.Code != http.StatusForbidden {
+		t.Errorf("POST /generate without token status = %d, want %d",
+			rec.Code, http.StatusForbidden)
+	}
+}
+
+// TestGeneratePostAcceptedWithValidToken verifies that POST /generate
+// succeeds with a valid session and a matching CSRF cookie and token.
+func TestGeneratePostAcceptedWithValidToken(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newCSRFTestRouter(t)
+
+	sessionCookie := newSessionCookie(t, h)
+
+	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
+	cookies = append(cookies, sessionCookie)
+
+	rec := postForm(srv, "/generate", cookies, url.Values{
+		"url":          {"https://example.com/a.jpg"},
+		"format":       {"jpeg"},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("POST /generate with valid token status = %d, want %d",
+			rec.Code, http.StatusOK)
+	}
+
+	if !strings.Contains(rec.Body.String(), "/v1/e/") {
+		t.Error("generator response did not contain a generated URL")
+	}
+}
+
+// newSessionCookie creates an authenticated session cookie via the
+// handler's session manager.
+func newSessionCookie(t *testing.T, h *Handlers) *http.Cookie {
+	t.Helper()
+
+	rec := httptest.NewRecorder()
+
+	err := h.sessMgr.CreateSession(rec)
+	if err != nil {
+		t.Fatalf("CreateSession() error = %v", err)
+	}
+
+	for _, c := range rec.Result().Cookies() {
+		if c.Name == session.CookieName {
+			return c
+		}
+	}
+
+	t.Fatalf("session manager did not set a %q cookie", session.CookieName)
+
+	return nil
+}

internal/handlers/bodylimit.go

@@ -0,0 +1,45 @@
+package handlers
+
+import (
+	"errors"
+	"net/http"
+)
+
+// MaxFormBytes bounds the request body accepted on the HTML form POST
+// routes (POST / and POST /generate). The forms carry a handful of short
+// fields, so 1 MiB is generous while making the bound explicit rather than
+// resting on ParseForm's incidental 10 MB cap.
+const MaxFormBytes = 1 << 20 // 1 MiB
+
+// LimitBody returns middleware that caps the request body on POST requests
+// at maxBytes and rejects an oversized body with 413 Request Entity Too
+// Large.
+//
+// It parses the form here, before the CSRF middleware reads the token from
+// it. The CSRF middleware reads the token with PostFormValue, which
+// swallows a parse error, so if the body were only capped there an
+// oversized body would read as a missing token and be refused as 403. By
+// parsing under the cap first, an oversized body is refused as 413. A
+// successful parse is cached on the request, so the CSRF check and the
+// handler reuse it rather than reading the body again.
+func (s *Handlers) LimitBody(maxBytes int64) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Method == http.MethodPost {
+				r.Body = http.MaxBytesReader(w, r.Body, maxBytes)
+
+				err := r.ParseForm()
+
+				var tooLarge *http.MaxBytesError
+				if errors.As(err, &tooLarge) {
+					http.Error(w, "Request body too large",
+						http.StatusRequestEntityTooLarge)
+
+					return
+				}
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}

internal/handlers/bodylimit_internal_test.go

@@ -0,0 +1,177 @@
+package handlers
+
+import (
+	"log/slog"
+	"net/http"
+	"net/url"
+	"strings"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// Form field names and a throwaway source image URL for the body-limit
+// tests.
+const (
+	sourceURLField = "url"
+	testSourceURL  = "https://example.com/a.jpg"
+)
+
+// newBodyLimitTestRouter mirrors the production wiring for the form POST
+// routes (see server.SetupRoutes): LimitBody sits in front of the CSRF
+// middleware, which sits in front of the handlers. maxBytes is the body
+// cap under test, so a test can trip the limit with a small body.
+func newBodyLimitTestRouter(
+	t *testing.T, maxBytes int64,
+) (*Handlers, http.Handler) {
+	t.Helper()
+
+	cfg := &config.Config{SigningKey: testSigningKey, Debug: true}
+
+	sessMgr, err := session.NewManager(testSigningKey)
+	if err != nil {
+		t.Fatalf("session.NewManager() error = %v", err)
+	}
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	protect, err := newCSRFProtect(testSigningKey, cfg.Debug)
+	if err != nil {
+		t.Fatalf("newCSRFProtect() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:         slog.New(slog.DiscardHandler),
+		config:      cfg,
+		sessMgr:     sessMgr,
+		encGen:      encGen,
+		csrfProtect: protect,
+	}
+
+	r := chi.NewRouter()
+	r.Group(func(r chi.Router) {
+		r.Use(h.LimitBody(maxBytes))
+		r.Use(h.CSRF())
+		r.Get("/", h.HandleRoot())
+		r.Post("/", h.HandleRoot())
+		r.Post("/generate", h.HandleGenerateURL())
+	})
+
+	return h, r
+}
+
+// TestOversizedLoginPostRejectedBeforeCSRF is the core regression: an
+// oversized POST / carrying an otherwise valid CSRF cookie and token must
+// be rejected with 413. If the body limit ran after CSRF, the truncated
+// body would read as a missing token and return 403; if it ran after the
+// handler, a valid token would return 303. Getting 413 proves the limit
+// fires before CSRF parses the form.
+func TestOversizedLoginPostRejectedBeforeCSRF(t *testing.T) {
+	t.Parallel()
+
+	_, srv := newBodyLimitTestRouter(t, 16)
+
+	cookies, token := csrfCredentials(t, srv, nil)
+
+	rec := postForm(srv, "/", cookies, url.Values{
+		loginKeyField:  {testSigningKey},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("oversized POST / status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+}
+
+// TestOversizedGeneratePostRejectedBeforeCSRF is the same regression for
+// POST /generate, which also parses a form behind CSRF.
+func TestOversizedGeneratePostRejectedBeforeCSRF(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newBodyLimitTestRouter(t, 16)
+
+	sessionCookie := newSessionCookie(t, h)
+
+	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
+	cookies = append(cookies, sessionCookie)
+
+	rec := postForm(srv, "/generate", cookies, url.Values{
+		sourceURLField: {testSourceURL},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Errorf("oversized POST /generate status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+}
+
+// TestWithinLimitLoginPostSucceeds verifies the limit does not disturb a
+// normal request: under the production cap, a valid login still parses and
+// establishes a session (303). This guards against the body limit
+// consuming or corrupting the form the CSRF check and handler depend on.
+func TestWithinLimitLoginPostSucceeds(t *testing.T) {
+	t.Parallel()
+
+	_, srv := newBodyLimitTestRouter(t, MaxFormBytes)
+
+	cookies, token := csrfCredentials(t, srv, nil)
+
+	rec := postForm(srv, "/", cookies, url.Values{
+		loginKeyField:  {testSigningKey},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("within-limit POST / status = %d, want %d",
+			rec.Code, http.StatusSeeOther)
+	}
+
+	var authed bool
+
+	for _, c := range rec.Result().Cookies() {
+		if c.Name == session.CookieName && c.Value != "" {
+			authed = true
+		}
+	}
+
+	if !authed {
+		t.Error("within-limit valid login did not set a session cookie")
+	}
+}
+
+// TestWithinLimitGeneratePostSucceeds is the same non-regression check for
+// POST /generate.
+func TestWithinLimitGeneratePostSucceeds(t *testing.T) {
+	t.Parallel()
+
+	h, srv := newBodyLimitTestRouter(t, MaxFormBytes)
+
+	sessionCookie := newSessionCookie(t, h)
+
+	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
+	cookies = append(cookies, sessionCookie)
+
+	rec := postForm(srv, "/generate", cookies, url.Values{
+		sourceURLField: {testSourceURL},
+		"format":       {"jpeg"},
+		csrfTokenField: {token},
+	})
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("within-limit POST /generate status = %d, want %d",
+			rec.Code, http.StatusOK)
+	}
+
+	if !strings.Contains(rec.Body.String(), "/v1/e/") {
+		t.Error("within-limit generate response did not contain a generated URL")
+	}
+}

internal/handlers/csrf.go

@@ -0,0 +1,65 @@
+package handlers
+
+import (
+	"html/template"
+	"net/http"
+
+	"github.com/gorilla/csrf"
+
+	"sneak.berlin/go/pixa/internal/seal"
+)
+
+// csrfKeySalt provides domain separation for the CSRF authentication key,
+// derived from the signing key so tokens survive restarts without extra
+// configuration and never reuse the session or encrypted-URL key material.
+const csrfKeySalt = "pixa-csrf-v1"
+
+// newCSRFProtect builds the CSRF-protection middleware for the
+// state-mutating HTML form routes. The token lives in its own cookie,
+// independent of the session cookie, so it also protects the login POST
+// where no session exists yet (login CSRF).
+//
+// When plaintext is true (local HTTP development), requests are marked
+// plaintext so the library neither demands an https Referer nor sets a
+// Secure cookie the browser would withhold over http. In production the
+// service runs behind a TLS-terminating proxy, so plaintext is false and
+// the library enforces its https Referer origin check.
+func newCSRFProtect(
+	signingKey string, plaintext bool,
+) (func(http.Handler) http.Handler, error) {
+	key, err := seal.DeriveKey([]byte(signingKey), csrfKeySalt)
+	if err != nil {
+		return nil, err
+	}
+
+	protect := csrf.Protect(
+		key[:],
+		csrf.Path("/"),
+		csrf.Secure(!plaintext),
+		csrf.SameSite(csrf.SameSiteStrictMode),
+	)
+
+	if !plaintext {
+		return protect, nil
+	}
+
+	return func(next http.Handler) http.Handler {
+		protected := protect(next)
+
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			protected.ServeHTTP(w, csrf.PlaintextHTTPRequest(r))
+		})
+	}, nil
+}
+
+// CSRF returns the CSRF-protection middleware for the login and generator
+// form routes.
+func (s *Handlers) CSRF() func(http.Handler) http.Handler {
+	return s.csrfProtect
+}
+
+// csrfField returns the hidden form input carrying the CSRF token for the
+// given request, to be embedded verbatim in a rendered form.
+func csrfField(r *http.Request) template.HTML {
+	return csrf.TemplateField(r)
+}

internal/handlers/handlers.go

@@ -39,15 +39,22 @@ type Handlers struct {
 	imgCache    *imgcache.Cache
 	sessMgr     *session.Manager
 	encGen      *encurl.Generator
+	csrfProtect func(http.Handler) http.Handler
 }
 
 // New creates a new Handlers instance.
 func New(lc fx.Lifecycle, params Params) (*Handlers, error) {
+	csrfProtect, err := newCSRFProtect(params.Config.SigningKey, params.Config.Debug)
+	if err != nil {
+		return nil, err
+	}
+
 	s := &Handlers{
 		log:         params.Logger.Get(),
 		hc:          params.Healthcheck,
 		db:          params.Database,
 		config:      params.Config,
+		csrfProtect: csrfProtect,
 	}
 
 	lc.Append(fx.Hook{
@@ -104,6 +111,8 @@ func (s *Handlers) initImageService() error {
 		fetcherCfg.MaxConnectionsPerHost = s.config.UpstreamConnectionsPerHost
 	}
 
+	fetcherCfg.BlockedNetworks = s.config.BlockedNetworks
+
 	// Create the service
 	svc, err := imgcache.NewService(&imgcache.ServiceConfig{
 		Cache:         cache,

internal/httpfetcher/blocked_networks_internal_test.go

@@ -0,0 +1,111 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"net/netip"
+	"testing"
+)
+
+// TestIsPrivateIPBlocksSpecialRanges covers the internal and special-use
+// ranges added to the built-in blocklist, in IPv4, IPv6, and IPv4-mapped
+// forms, alongside public controls that must stay reachable.
+func TestIsPrivateIPBlocksSpecialRanges(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		ip   string
+		want bool
+	}{
+		{"cgnat-low", "100.64.0.1", true},
+		{"cgnat-high", "100.127.255.254", true},
+		{"ietf-protocol", "192.0.0.1", true},
+		{"benchmark-low", "198.18.0.1", true},
+		{"benchmark-high", "198.19.255.254", true},
+		{"nat64", "64:ff9b::1", true},
+		{"nat64-embeds-private", "64:ff9b::a00:1", true}, // maps 10.0.0.1
+		{"ipv4-mapped-private", "::ffff:10.0.0.1", true},
+		{"cloud-metadata", "169.254.169.254", true},
+		{"public-v4", "8.8.8.8", false},
+		{"test-net-1-public", testPublicHost, false}, // TEST-NET-1, stays public
+		{"public-v6", "2001:4860:4860::8888", false},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			ip := net.ParseIP(tc.ip)
+			if ip == nil {
+				t.Fatalf("failed to parse IP %q", tc.ip)
+			}
+
+			got := isPrivateIP(ip)
+			if got != tc.want {
+				t.Errorf("isPrivateIP(%q) = %v, want %v", tc.ip, got, tc.want)
+			}
+		})
+	}
+}
+
+// transportOf returns the *http.Transport backing a fetcher, so a test can
+// exercise the SSRF-safe dialer New installed with the operator blocklist.
+func transportOf(t *testing.T, f *HTTPFetcher) *http.Transport {
+	t.Helper()
+
+	transport, ok := f.client.Transport.(*http.Transport)
+	if !ok {
+		t.Fatalf("transport is %T, want *http.Transport", f.client.Transport)
+	}
+
+	return transport
+}
+
+// TestDialerEnforcesBlockedNetworks proves an operator-supplied
+// blocked_networks entry is enforced by the dialer, in addition to the
+// built-in ranges, while an address outside both stays dialable.
+func TestDialerEnforcesBlockedNetworks(t *testing.T) {
+	t.Parallel()
+
+	cfg := DefaultConfig()
+	// TEST-NET-2 (198.51.100.0/24) is public to the built-in check, so
+	// blocking it can only come from the operator-supplied list.
+	cfg.BlockedNetworks = []netip.Prefix{netip.MustParsePrefix("198.51.100.0/24")}
+
+	transport := transportOf(t, New(cfg))
+
+	blocked := []string{
+		"198.51.100.5:80", // operator-supplied range
+		"10.0.0.5:80",     // built-in RFC 1918, still enforced
+		"100.64.0.1:80",   // built-in CGNAT range
+	}
+
+	for _, addr := range blocked {
+		t.Run("blocked/"+addr, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := transport.DialContext(context.Background(), "tcp", addr)
+			if !errors.Is(err, ErrSSRFBlocked) {
+				t.Errorf("DialContext(%q) = %v, want ErrSSRFBlocked", addr, err)
+			}
+		})
+	}
+
+	t.Run("public-not-blocked", func(t *testing.T) {
+		t.Parallel()
+
+		// A cancelled context makes the dial fail without touching the
+		// network; the point is only that a public literal outside every
+		// blocked range is not SSRF-blocked.
+		ctx, cancel := context.WithCancel(context.Background())
+		cancel()
+
+		_, err := transport.DialContext(ctx, "tcp", testPublicHost+":80")
+		if errors.Is(err, ErrSSRFBlocked) {
+			t.Errorf("public target SSRF-blocked with operator list set: %v", err)
+		}
+	})
+}

internal/httpfetcher/fetch_internal_test.go

@@ -0,0 +1,421 @@
+package httpfetcher
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// testPublicHost is a TEST-NET-1 (RFC 5737) literal. isPrivateIP treats it as
+// public, so validateURL and the redirect check accept it with no DNS lookup,
+// while the recording dialer routes it to the local httptest server. The
+// address is reserved for documentation and is never routed on the network.
+const testPublicHost = "192.0.2.10"
+
+// imagePayload is the body served by the fake upstream's image route.
+const imagePayload = "fake-jpeg-bytes"
+
+// errUnexpectedDial reports a dial to any host other than testPublicHost, which
+// would mean SSRF protection let a forbidden target reach the transport.
+var errUnexpectedDial = errors.New("unexpected dial target")
+
+// upstreamURL builds a fetch URL on the fake public host for the given path.
+func upstreamURL(path string) string {
+	return "http://" + testPublicHost + path
+}
+
+// recordingDialer records every address the transport asks it to dial and
+// routes connections for testPublicHost to a real local server, so the SSRF
+// checks run against a public-looking host while bytes go to httptest.
+type recordingDialer struct {
+	target string
+
+	mu     sync.Mutex
+	dialed []string
+}
+
+func (d *recordingDialer) dialContext(
+	ctx context.Context,
+	network, addr string,
+) (net.Conn, error) {
+	d.mu.Lock()
+	d.dialed = append(d.dialed, addr)
+	d.mu.Unlock()
+
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		return nil, err
+	}
+
+	if host != testPublicHost {
+		return nil, fmt.Errorf("%w: %s", errUnexpectedDial, addr)
+	}
+
+	var dialer net.Dialer
+
+	return dialer.DialContext(ctx, network, d.target)
+}
+
+// dialedAddrs returns a copy of the addresses the dialer was asked to reach.
+func (d *recordingDialer) dialedAddrs() []string {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+
+	return slices.Clone(d.dialed)
+}
+
+// startUpstream launches a fake upstream with the routes the fetch tests
+// exercise and stops it when the test finishes.
+func startUpstream(t *testing.T) *httptest.Server {
+	t.Helper()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("/image", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", contentTypeJPEG)
+		_, _ = io.WriteString(w, imagePayload)
+	})
+	mux.HandleFunc("/status/500", func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+	})
+	mux.HandleFunc("/html", func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		_, _ = io.WriteString(w, "<html></html>")
+	})
+	mux.HandleFunc("/redirect/private", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "http://169.254.169.254/latest/meta-data/", http.StatusFound)
+	})
+	mux.HandleFunc("/redirect/public", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/image", http.StatusFound)
+	})
+	mux.HandleFunc("/redirect/chain", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/redirect/hop", http.StatusFound)
+	})
+	mux.HandleFunc("/redirect/hop", func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/image", http.StatusFound)
+	})
+
+	srv := httptest.NewServer(mux)
+	t.Cleanup(srv.Close)
+
+	return srv
+}
+
+// newServerFetcher builds a fetcher whose transport routes testPublicHost to
+// srv, leaving the real SSRF validation and redirect checks in place.
+func newServerFetcher(
+	t *testing.T,
+	srv *httptest.Server,
+	cfg *Config,
+) (*HTTPFetcher, *recordingDialer) {
+	t.Helper()
+
+	if cfg == nil {
+		cfg = DefaultConfig()
+	}
+
+	cfg.AllowHTTP = true
+
+	f := New(cfg)
+
+	transport, ok := f.client.Transport.(*http.Transport)
+	if !ok {
+		t.Fatalf("transport is %T, want *http.Transport", f.client.Transport)
+	}
+
+	dialer := &recordingDialer{target: srv.Listener.Addr().String()}
+	transport.DialContext = dialer.dialContext
+
+	return f, dialer
+}
+
+// testContext returns a context cancelled when the test ends, bounding any
+// fetch that would otherwise block on a leaked semaphore slot.
+func testContext(t *testing.T) context.Context {
+	t.Helper()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	t.Cleanup(cancel)
+
+	return ctx
+}
+
+// fetchImage fetches path from the fake upstream and fails on error.
+func fetchImage(t *testing.T, f *HTTPFetcher, path string) *FetchResult {
+	t.Helper()
+
+	res, err := f.Fetch(testContext(t), upstreamURL(path))
+	if err != nil {
+		t.Fatalf("Fetch(%s) error = %v", path, err)
+	}
+
+	return res
+}
+
+// fetchExpectError fetches path and fails unless Fetch returns an error.
+func fetchExpectError(t *testing.T, f *HTTPFetcher, path string) error {
+	t.Helper()
+
+	res, err := f.Fetch(testContext(t), upstreamURL(path))
+	if err == nil {
+		_ = res.Content.Close()
+
+		t.Fatalf("Fetch(%s) = nil error, want an error", path)
+	}
+
+	return err
+}
+
+// fetchBody fetches path and returns the fully read, closed response body.
+func fetchBody(t *testing.T, f *HTTPFetcher, path string) string {
+	t.Helper()
+
+	res := fetchImage(t, f, path)
+	defer func() { _ = res.Content.Close() }()
+
+	data, err := io.ReadAll(res.Content)
+	if err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+
+	return string(data)
+}
+
+// semLen reports how many per-host semaphore slots are currently held.
+func semLen(f *HTTPFetcher, host string) int {
+	return len(f.getHostSemaphore(host))
+}
+
+func TestFetchRedirectToPrivateIPBlocked(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, dialer := newServerFetcher(t, srv, nil)
+
+	_, err := f.Fetch(testContext(t), upstreamURL("/redirect/private"))
+	if !errors.Is(err, ErrSSRFBlocked) {
+		t.Fatalf("Fetch() error = %v, want ErrSSRFBlocked", err)
+	}
+
+	for _, addr := range dialer.dialedAddrs() {
+		if strings.Contains(addr, "169.254.169.254") {
+			t.Errorf("dialer connected to the private redirect target: %s", addr)
+		}
+	}
+}
+
+func TestFetchRedirectToPublicSucceeds(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, _ := newServerFetcher(t, srv, nil)
+
+	if body := fetchBody(t, f, "/redirect/public"); body != imagePayload {
+		t.Errorf("body = %q, want %q", body, imagePayload)
+	}
+}
+
+func TestFetchRedirectChainSucceeds(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, _ := newServerFetcher(t, srv, nil)
+
+	if body := fetchBody(t, f, "/redirect/chain"); body != imagePayload {
+		t.Errorf("body = %q, want %q", body, imagePayload)
+	}
+}
+
+func TestFetchRejectsNon2xx(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, _ := newServerFetcher(t, srv, nil)
+
+	err := fetchExpectError(t, f, "/status/500")
+	if !errors.Is(err, ErrUpstreamError) {
+		t.Fatalf("Fetch() error = %v, want ErrUpstreamError", err)
+	}
+}
+
+func TestFetchRejectsDisallowedContentType(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+	f, _ := newServerFetcher(t, srv, nil)
+
+	err := fetchExpectError(t, f, "/html")
+	if !errors.Is(err, ErrInvalidContentType) {
+		t.Fatalf("Fetch() error = %v, want ErrInvalidContentType", err)
+	}
+}
+
+func TestFetchMaxResponseSizeEnforced(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxResponseSize = 8
+
+	f, _ := newServerFetcher(t, srv, cfg)
+
+	res := fetchImage(t, f, "/image")
+	defer func() { _ = res.Content.Close() }()
+
+	data, err := io.ReadAll(res.Content)
+	if !errors.Is(err, ErrResponseTooLarge) {
+		t.Fatalf("read error = %v, want ErrResponseTooLarge", err)
+	}
+
+	if int64(len(data)) > cfg.MaxResponseSize {
+		t.Errorf("read %d bytes, exceeds limit %d", len(data), cfg.MaxResponseSize)
+	}
+}
+
+func TestFetchSemaphoreReleasedOnError(t *testing.T) {
+	t.Parallel()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnectionsPerHost = 1
+
+	f, _ := newServerFetcher(t, srv, cfg)
+
+	err := fetchExpectError(t, f, "/status/500")
+	if !errors.Is(err, ErrUpstreamError) {
+		t.Fatalf("Fetch() error = %v, want ErrUpstreamError", err)
+	}
+
+	if held := semLen(f, testPublicHost); held != 0 {
+		t.Fatalf("semaphore slot leaked after error: %d held", held)
+	}
+
+	// One slot per host: this fetch proceeds only if the slot was released.
+	res := fetchImage(t, f, "/image")
+	_ = res.Content.Close()
+}
+
+// assertSlotReleasedByClose fetches an image over a one-slot host, hands the
+// open result to consume, and asserts the slot is held before and freed after,
+// then that a follow-up fetch can still acquire it.
+func assertSlotReleasedByClose(
+	t *testing.T,
+	consume func(*testing.T, *FetchResult),
+) {
+	t.Helper()
+
+	srv := startUpstream(t)
+
+	cfg := DefaultConfig()
+	cfg.MaxConnectionsPerHost = 1
+
+	f, _ := newServerFetcher(t, srv, cfg)
+
+	res := fetchImage(t, f, "/image")
+	if held := semLen(f, testPublicHost); held != 1 {
+		t.Fatalf("slot not held while body is open: %d held", held)
+	}
+
+	consume(t, res)
+
+	if held := semLen(f, testPublicHost); held != 0 {
+		t.Fatalf("slot not released after close: %d held", held)
+	}
+
+	next := fetchImage(t, f, "/image")
+	_ = next.Content.Close()
+}
+
+func TestFetchSemaphoreReleasedOnBodyClose(t *testing.T) {
+	t.Parallel()
+
+	assertSlotReleasedByClose(t, func(t *testing.T, res *FetchResult) {
+		t.Helper()
+
+		_, err := io.ReadAll(res.Content)
+		if err != nil {
+			t.Fatalf("read body: %v", err)
+		}
+
+		err = res.Content.Close()
+		if err != nil {
+			t.Fatalf("close body: %v", err)
+		}
+	})
+}
+
+func TestFetchSemaphoreReleasedOnPartialReadClose(t *testing.T) {
+	t.Parallel()
+
+	assertSlotReleasedByClose(t, func(t *testing.T, res *FetchResult) {
+		t.Helper()
+
+		buf := make([]byte, 1)
+
+		_, err := res.Content.Read(buf)
+		if err != nil {
+			t.Fatalf("partial read: %v", err)
+		}
+
+		err = res.Content.Close()
+		if err != nil {
+			t.Fatalf("close body: %v", err)
+		}
+	})
+}
+
+// The dial-time re-resolution in ssrfSafeDialer is what closes the DNS
+// rebinding window: even if validateURL saw a public answer earlier, the
+// dialer independently re-checks the address it is about to connect to. A full
+// rebinding simulation (a resolver returning public, then private) would mean
+// replacing the global net.DefaultResolver with a fake DNS server, which is
+// heavyweight and unsafe to mutate under parallel -race tests. The property is
+// proven directly here instead: the dialer rejects a private target outright,
+// which is exactly the check that fires when a validated host later resolves
+// to a private address.
+func TestSSRFSafeDialerBlocksPrivateTarget(t *testing.T) {
+	t.Parallel()
+
+	for _, addr := range []string{
+		"169.254.169.254:80", // link-local (cloud metadata)
+		"127.0.0.1:80",       // loopback
+		"10.0.0.5:80",        // RFC 1918 private
+	} {
+		t.Run(addr, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := ssrfSafeDialer(context.Background(), "tcp", addr)
+			if !errors.Is(err, ErrSSRFBlocked) {
+				t.Errorf("ssrfSafeDialer(%q) = %v, want ErrSSRFBlocked", addr, err)
+			}
+		})
+	}
+}
+
+func TestSSRFSafeDialerAllowsPublicTarget(t *testing.T) {
+	t.Parallel()
+
+	// A cancelled context makes the dial fail immediately without touching the
+	// network; the point is only that a public literal is not SSRF-blocked.
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	_, err := ssrfSafeDialer(ctx, "tcp", testPublicHost+":80")
+	if err == nil {
+		t.Fatal("expected a dial error for an unreachable public target")
+	}
+
+	if errors.Is(err, ErrSSRFBlocked) {
+		t.Errorf("public target was SSRF-blocked: %v", err)
+	}
+}

internal/httpfetcher/httpfetcher.go

@@ -11,6 +11,7 @@ import (
 	"net"
 	"net/http"
 	"net/http/httptrace"
+	"net/netip"
 	neturl "net/url"
 	"slices"
 	"strings"
@@ -46,6 +47,20 @@ const (
 	localhostIPv6 = "::1"
 )
 
+// builtinBlockedPrefixes are internal or special-use ranges that Go's
+// net.IP predicates (IsPrivate, IsLinkLocalUnicast, and the like) do not
+// already cover. They are always blocked, in addition to any
+// operator-supplied networks. IPv4-mapped IPv6 addresses are unmapped
+// before matching, so these IPv4 ranges are caught in both forms.
+//
+//nolint:gochecknoglobals // immutable built-in blocklist
+var builtinBlockedPrefixes = []netip.Prefix{
+	netip.MustParsePrefix("100.64.0.0/10"), // RFC 6598 CGNAT / carrier-grade NAT
+	netip.MustParsePrefix("192.0.0.0/24"),  // RFC 6890 IETF protocol assignments
+	netip.MustParsePrefix("198.18.0.0/15"), // RFC 2544 benchmarking range
+	netip.MustParsePrefix("64:ff9b::/96"),  // RFC 6052 NAT64 (maps onto IPv4)
+}
+
 // Fetcher errors.
 var (
 	ErrSSRFBlocked        = errors.New("request blocked: private or internal IP")
@@ -107,6 +122,9 @@ type Config struct {
 	AllowHTTP bool
 	// MaxConnectionsPerHost limits concurrent connections to each upstream host.
 	MaxConnectionsPerHost int
+	// BlockedNetworks are operator-supplied CIDR ranges refused by the
+	// dialer, in addition to the always-enforced built-in ranges.
+	BlockedNetworks []netip.Prefix
 }
 
 // DefaultConfig returns a Config with sensible defaults.
@@ -142,9 +160,13 @@ func New(config *Config) *HTTPFetcher {
 		config = DefaultConfig()
 	}
 
-	// Create transport with SSRF-safe dialer
+	// Create transport with SSRF-safe dialer. The dialer re-resolves and
+	// re-checks at connect time (closing the DNS-rebinding window) against
+	// both the built-in ranges and the operator-supplied blocklist.
 	transport := &http.Transport{
-		DialContext:         ssrfSafeDialer,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			return dialSSRFSafe(ctx, network, addr, config.BlockedNetworks)
+		},
 		TLSHandshakeTimeout: DefaultTLSTimeout,
 		MaxIdleConns:        DefaultMaxIdleConns,
 		IdleConnTimeout:     DefaultIdleConnTimeout,
@@ -451,11 +473,53 @@ func isPrivateIP(ip net.IP) bool {
 		}
 	}
 
-	return false
+	// Special-use ranges the net.IP predicates above do not cover.
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return true
 	}
 
-// ssrfSafeDialer is a custom dialer that validates IP addresses before connecting.
+	addr = addr.Unmap()
+
+	return slices.ContainsFunc(builtinBlockedPrefixes, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr)
+	})
+}
+
+// isBlockedIP reports whether ip is refused, either by the built-in
+// internal-range check or by one of the operator-supplied prefixes.
+func isBlockedIP(ip net.IP, blocked []netip.Prefix) bool {
+	if isPrivateIP(ip) {
+		return true
+	}
+
+	addr, ok := netip.AddrFromSlice(ip)
+	if !ok {
+		return true
+	}
+
+	addr = addr.Unmap()
+
+	return slices.ContainsFunc(blocked, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr)
+	})
+}
+
+// ssrfSafeDialer validates IP addresses against the built-in blocked ranges
+// before connecting. New wraps dialSSRFSafe with the operator-supplied
+// blocklist; this entry point enforces the built-in ranges alone.
 func ssrfSafeDialer(ctx context.Context, network, addr string) (net.Conn, error) {
+	return dialSSRFSafe(ctx, network, addr, nil)
+}
+
+// dialSSRFSafe re-resolves addr and refuses to connect to any built-in
+// internal range or operator-supplied blocked prefix, closing the
+// DNS-rebinding window at connect time.
+func dialSSRFSafe(
+	ctx context.Context,
+	network, addr string,
+	blocked []netip.Prefix,
+) (net.Conn, error) {
 	host, port, err := net.SplitHostPort(addr)
 	if err != nil {
 		return nil, err
@@ -468,9 +532,11 @@ func ssrfSafeDialer(ctx context.Context, network, addr string) (net.Conn, error)
 	}
 
 	// Check all resolved IPs
-	if slices.ContainsFunc(ips, isPrivateIP) {
+	for _, ip := range ips {
+		if isBlockedIP(ip, blocked) {
 			return nil, ErrSSRFBlocked
 		}
+	}
 
 	// Connect using the first valid IP
 	var dialer net.Dialer

internal/imgcache/service.go

@@ -452,8 +452,6 @@ func signatureRequest(req *ImageRequest) *signature.Request {
 		Width:       req.Size.Width,
 		Height:      req.Size.Height,
 		Format:      string(req.Format),
-		Quality:     req.Quality,
-		FitMode:     string(req.FitMode),
 		Signature:   req.Signature,
 		Expires:     req.Expires,
 	}

internal/middleware/client_ip_internal_test.go

@@ -0,0 +1,118 @@
+package middleware
+
+import (
+	"bytes"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/clientip"
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// testForwardedClient is the client address the proxy forwards.
+const testForwardedClient = "203.0.113.7"
+
+// newTestMiddleware builds a Middleware whose resolver trusts the given
+// CIDRs and whose logger writes JSON to buf.
+func newTestMiddleware(t *testing.T, buf *bytes.Buffer, trusted ...string) *Middleware {
+	t.Helper()
+
+	prefixes := make([]netip.Prefix, 0, len(trusted))
+
+	for _, c := range trusted {
+		p, err := netip.ParsePrefix(c)
+		if err != nil {
+			t.Fatalf("netip.ParsePrefix(%q) error = %v", c, err)
+		}
+
+		prefixes = append(prefixes, p)
+	}
+
+	return &Middleware{
+		log:      slog.New(slog.NewJSONHandler(buf, nil)),
+		config:   &config.Config{TrustedProxies: prefixes},
+		clientIP: clientip.NewResolver(prefixes),
+	}
+}
+
+// TestClientIPMiddlewareStoresResolvedIP verifies the ClientIP middleware
+// puts the resolved address into the request context for a trusted and an
+// untrusted peer.
+func TestClientIPMiddlewareStoresResolvedIP(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		remoteAddr string
+		forwarded  string
+		want       string
+	}{
+		{
+			name:       "trusted peer honors forwarded client",
+			remoteAddr: "10.0.0.1:5000",
+			forwarded:  testForwardedClient,
+			want:       testForwardedClient,
+		},
+		{
+			name:       "untrusted peer ignores forwarded header",
+			remoteAddr: "198.51.100.9:5000",
+			forwarded:  testForwardedClient,
+			want:       "198.51.100.9",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			mw := newTestMiddleware(t, &bytes.Buffer{}, "10.0.0.0/8")
+
+			var got string
+
+			handler := mw.ClientIP()(http.HandlerFunc(
+				func(_ http.ResponseWriter, r *http.Request) {
+					got = clientip.FromContext(r.Context())
+				}))
+
+			req := httptest.NewRequestWithContext(
+				t.Context(), http.MethodGet, "/", nil)
+			req.RemoteAddr = tt.remoteAddr
+			req.Header.Set("X-Forwarded-For", tt.forwarded)
+
+			handler.ServeHTTP(httptest.NewRecorder(), req)
+
+			if got != tt.want {
+				t.Errorf("client IP in context = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestLoggingUsesResolvedClientIP verifies the logging middleware records
+// the resolved forwarded client IP rather than the proxy peer address.
+func TestLoggingUsesResolvedClientIP(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	mw := newTestMiddleware(t, &buf, "10.0.0.0/8")
+
+	handler := mw.ClientIP()(mw.Logging()(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		})))
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/", nil)
+	req.RemoteAddr = "10.0.0.1:5000"
+	req.Header.Set("X-Forwarded-For", testForwardedClient)
+
+	handler.ServeHTTP(httptest.NewRecorder(), req)
+
+	if !strings.Contains(buf.String(), `"remoteIP":"`+testForwardedClient+`"`) {
+		t.Errorf("log output missing resolved client IP; got %q", buf.String())
+	}
+}

internal/middleware/middleware.go

@@ -3,7 +3,6 @@ package middleware
 
 import (
 	"log/slog"
-	"net"
 	"net/http"
 	"time"
 
@@ -14,6 +13,7 @@ import (
 	ghmm "github.com/slok/go-http-metrics/middleware"
 	"github.com/slok/go-http-metrics/middleware/std"
 	"go.uber.org/fx"
+	"sneak.berlin/go/pixa/internal/clientip"
 	"sneak.berlin/go/pixa/internal/config"
 	"sneak.berlin/go/pixa/internal/logger"
 )
@@ -21,6 +21,33 @@ import (
 // CORSMaxAgeSeconds is the max age for CORS preflight cache (24 hours).
 const CORSMaxAgeSeconds = 86400
 
+// HSTSValue is the Strict-Transport-Security header value: one year with
+// includeSubDomains. Emitted unconditionally even though pixa listens plain
+// HTTP behind a TLS-terminating proxy; browsers ignore an HSTS header received
+// over plaintext (RFC 6797 section 8.1), so it never lies about the connection,
+// and emitting it here avoids trusting a forwarded-proto header.
+const HSTSValue = "max-age=31536000; includeSubDomains"
+
+// ContentSecurityPolicyValue is the Content-Security-Policy header value.
+// default-src 'self' is the baseline and frame-ancestors 'none' is the primary
+// clickjacking control. 'unsafe-inline' is required in script-src and style-src
+// because the served templates carry inline onclick handlers (generator page)
+// and the bundled Tailwind asset injects a runtime <style> element; dropping it
+// needs template changes outside this issue's scope.
+const ContentSecurityPolicyValue = "default-src 'self'; " +
+	"script-src 'self' 'unsafe-inline'; " +
+	"style-src 'self' 'unsafe-inline'; " +
+	"object-src 'none'; " +
+	"base-uri 'self'; " +
+	"form-action 'self'; " +
+	"frame-ancestors 'none'"
+
+// PermissionsPolicyValue is the Permissions-Policy header value. Every listed
+// feature is denied because pixa uses none of them.
+const PermissionsPolicyValue = "accelerometer=(), autoplay=(), camera=(), " +
+	"display-capture=(), geolocation=(), gyroscope=(), magnetometer=(), " +
+	"microphone=(), payment=(), usb=()"
+
 // Params defines dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -33,6 +60,7 @@ type Params struct {
 type Middleware struct {
 	log      *slog.Logger
 	config   *config.Config
+	clientIP *clientip.Resolver
 }
 
 // New creates a new Middleware instance.
@@ -40,22 +68,24 @@ func New(_ fx.Lifecycle, params Params) (*Middleware, error) {
 	s := &Middleware{
 		log:      params.Logger.Get(),
 		config:   params.Config,
+		clientIP: clientip.NewResolver(params.Config.TrustedProxies),
 	}
 
 	return s, nil
 }
 
-func ipFromHostPort(hp string) string {
+// ClientIP returns a middleware that resolves the real client IP,
-	h, _, err := net.SplitHostPort(hp)
+// honoring X-Forwarded-For only from trusted proxies, and stores it in
-	if err != nil {
+// the request context for the logging middleware and handlers to read.
-		return ""
+func (s *Middleware) ClientIP() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			ip := s.clientIP.Resolve(
+				r.RemoteAddr, r.Header.Values(clientip.ForwardedForHeader))
+			ctx := clientip.WithClientIP(r.Context(), ip)
+			next.ServeHTTP(w, r.WithContext(ctx))
+		})
 	}
-
-	if len(h) > 0 && h[0] == '[' {
-		return h[1 : len(h)-1]
-	}
-
-	return h
 }
 
 type loggingResponseWriter struct {
@@ -100,7 +130,7 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 					"request_id", reqID,
 					"referer", r.Referer(),
 					"proto", r.Proto,
-					"remoteIP", ipFromHostPort(r.RemoteAddr),
+					"remoteIP", clientip.FromContext(ctx),
 					"status", lrw.statusCode,
 					"response_bytes", lrw.bytesWritten,
 					"latency_ms", latency.Milliseconds(),
@@ -164,6 +194,16 @@ func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 			// Disable XSS filtering (modern browsers don't need it, can cause issues)
 			w.Header().Set("X-XSS-Protection", "0")
 
+			// Force HTTPS on future visits (ignored by browsers over plaintext)
+			w.Header().Set("Strict-Transport-Security", HSTSValue)
+
+			// Restrict content sources; frame-ancestors is the primary
+			// clickjacking control, X-Frame-Options the legacy fallback
+			w.Header().Set("Content-Security-Policy", ContentSecurityPolicyValue)
+
+			// Deny browser features pixa does not use
+			w.Header().Set("Permissions-Policy", PermissionsPolicyValue)
+
 			next.ServeHTTP(w, r)
 		})
 	}

internal/middleware/middleware_internal_test.go

@@ -56,6 +56,61 @@ func TestSecurityHeaders(t *testing.T) {
 	}
 }
 
+func TestSecurityHeaders_PolicyHeaders(t *testing.T) {
+	t.Parallel()
+
+	cfg := &config.Config{}
+	mw := &Middleware{
+		log:    slog.Default(),
+		config: cfg,
+	}
+
+	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	})
+
+	handler := mw.SecurityHeaders()(testHandler)
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/test", nil)
+	rec := httptest.NewRecorder()
+
+	handler.ServeHTTP(rec, req)
+
+	tests := []struct {
+		header string
+		want   string
+	}{
+		{"Strict-Transport-Security", "max-age=31536000; includeSubDomains"},
+		{
+			"Content-Security-Policy",
+			"default-src 'self'; " +
+				"script-src 'self' 'unsafe-inline'; " +
+				"style-src 'self' 'unsafe-inline'; " +
+				"object-src 'none'; " +
+				"base-uri 'self'; " +
+				"form-action 'self'; " +
+				"frame-ancestors 'none'",
+		},
+		{
+			"Permissions-Policy",
+			"accelerometer=(), autoplay=(), camera=(), " +
+				"display-capture=(), geolocation=(), gyroscope=(), " +
+				"magnetometer=(), microphone=(), payment=(), usb=()",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.header, func(t *testing.T) {
+			t.Parallel()
+
+			got := rec.Header().Get(tt.header)
+			if got != tt.want {
+				t.Errorf("%s = %q, want %q", tt.header, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestSecurityHeaders_PreservesExistingHeaders(t *testing.T) {
 	t.Parallel()
 

internal/server/http.go

@@ -10,23 +10,39 @@ import (
 // HTTP server configuration constants.
 const (
 	HTTPReadTimeout = 30 * time.Second
+	// HTTPReadHeaderTimeout bounds the request-header read on its own,
+	// short, so a slowloris client dribbling headers is dropped well
+	// before it ties up a connection for the whole ReadTimeout window.
+	HTTPReadHeaderTimeout = 10 * time.Second
 	HTTPWriteTimeout      = 60 * time.Second
+	// HTTPIdleTimeout bounds how long an idle keep-alive connection is
+	// held open, so idle connections cannot accumulate without limit on a
+	// service targeting high concurrency.
+	HTTPIdleTimeout    = 120 * time.Second
 	HTTPMaxHeaderBytes = 8 << 10 // 8KB
 )
 
-func (s *Server) serveUntilShutdown() {
+// newHTTPServer builds the http.Server with the hardening timeouts and
-	listenAddr := fmt.Sprintf(":%d", s.config.Port)
+// limits applied. It is separate from serveUntilShutdown so the
-	s.httpServer = &http.Server{
+// configuration can be asserted in a test without binding a listener.
-		Addr:           listenAddr,
+func (s *Server) newHTTPServer() *http.Server {
+	return &http.Server{
+		Addr:              fmt.Sprintf(":%d", s.config.Port),
 		ReadTimeout:       HTTPReadTimeout,
+		ReadHeaderTimeout: HTTPReadHeaderTimeout,
 		WriteTimeout:      HTTPWriteTimeout,
+		IdleTimeout:       HTTPIdleTimeout,
 		MaxHeaderBytes:    HTTPMaxHeaderBytes,
 		Handler:           s,
 	}
+}
+
+func (s *Server) serveUntilShutdown() {
+	s.httpServer = s.newHTTPServer()
 
 	s.SetupRoutes()
 
-	s.log.Info("http begin listen", "listenaddr", listenAddr)
+	s.log.Info("http begin listen", "listenaddr", s.httpServer.Addr)
 
 	err := s.httpServer.ListenAndServe()
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {

internal/server/http_internal_test.go

@@ -0,0 +1,65 @@
+package server
+
+import (
+	"testing"
+	"time"
+
+	"sneak.berlin/go/pixa/internal/config"
+)
+
+// TestNewHTTPServerTimeouts verifies that the constructed http.Server
+// carries every hardening timeout wired onto it, including the slowloris
+// defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
+// guards against a field being defined but never set on the server, so
+// each assertion compares the server field to its constant.
+func TestNewHTTPServerTimeouts(t *testing.T) {
+	t.Parallel()
+
+	s := &Server{config: &config.Config{Port: 8080}}
+
+	srv := s.newHTTPServer()
+
+	fields := []struct {
+		name string
+		got  time.Duration
+		want time.Duration
+	}{
+		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
+		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
+		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
+		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
+	}
+
+	for _, f := range fields {
+		if f.got != f.want {
+			t.Errorf("%s = %v, want %v", f.name, f.got, f.want)
+		}
+	}
+
+	if srv.MaxHeaderBytes != HTTPMaxHeaderBytes {
+		t.Errorf("MaxHeaderBytes = %d, want %d",
+			srv.MaxHeaderBytes, HTTPMaxHeaderBytes)
+	}
+
+	if srv.Handler != s {
+		t.Error("Handler is not the server")
+	}
+}
+
+// TestHardeningTimeoutValues pins the intent behind the two new timeouts
+// without hard-coding brittle exact durations: the header-read phase is
+// bounded strictly shorter than the whole-request read (the slowloris
+// dribble), and idle keep-alive connections are bounded rather than held
+// open forever.
+func TestHardeningTimeoutValues(t *testing.T) {
+	t.Parallel()
+
+	if HTTPReadHeaderTimeout <= 0 || HTTPReadHeaderTimeout > HTTPReadTimeout {
+		t.Errorf("ReadHeaderTimeout = %v, want positive and <= ReadTimeout %v",
+			HTTPReadHeaderTimeout, HTTPReadTimeout)
+	}
+
+	if HTTPIdleTimeout <= 0 {
+		t.Errorf("IdleTimeout = %v, want positive bound", HTTPIdleTimeout)
+	}
+}

internal/server/routes.go

@@ -8,6 +8,7 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 
+	"sneak.berlin/go/pixa/internal/handlers"
 	"sneak.berlin/go/pixa/internal/static"
 )
 
@@ -17,6 +18,7 @@ func (s *Server) SetupRoutes() {
 
 	s.router.Use(middleware.Recoverer)
 	s.router.Use(middleware.RequestID)
+	s.router.Use(s.mw.ClientIP())
 	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.Logging())
 
@@ -44,11 +46,19 @@ func (s *Server) SetupRoutes() {
 	// Static files (Tailwind CSS, etc.)
 	s.router.Handle("/static/*", http.StripPrefix("/static/", static.Handler()))
 
-	// Login/generator UI
+	// Login/generator UI. The form routes carry CSRF protection; the
-	s.router.Get("/", s.h.HandleRoot())
+	// token cookie is independent of the session cookie, so it also
-	s.router.Post("/", s.h.HandleRoot())
+	// covers the login POST, where no session exists yet. LimitBody caps
+	// the POST body ahead of CSRF, which reads its token from that body.
+	s.router.Group(func(r chi.Router) {
+		r.Use(s.h.LimitBody(handlers.MaxFormBytes))
+		r.Use(s.h.CSRF())
+		r.Get("/", s.h.HandleRoot())
+		r.Post("/", s.h.HandleRoot())
+		r.Post("/generate", s.h.HandleGenerateURL())
+	})
+
 	s.router.Get("/logout", s.h.HandleLogout())
-	s.router.Post("/generate", s.h.HandleGenerateURL())
 
 	// Main image proxy route
 	// /v1/image/<host>/<path>/<width>x<height>.<format>

internal/signature/golden_qualityfit_test.go

@@ -1,122 +0,0 @@
-package signature_test
-
-import (
-	"testing"
-	"time"
-
-	"sneak.berlin/go/pixa/internal/signature"
-)
-
-// Fixed inputs for the quality/fit golden vectors. They are independent of
-// the constants in golden_test.go so this file pins the current signed
-// format on its own.
-const (
-	qfSigningKey        = "golden-test-key"
-	qfExpiresUnix int64 = 1704067200 // 2024-01-01T00:00:00Z
-	qfFitCover          = "cover"
-	qfFitContain        = "contain"
-)
-
-type qualityFitGoldenVector struct {
-	name string
-	req  signature.Request
-	// wantSignature is the exact base64url (RFC 4648 URL-safe, padded)
-	// HMAC-SHA256 signature for the request with Expires set to
-	// qfExpiresUnix, under the signed format
-	// "host:path:query:width:height:format:expiration:quality:fit".
-	wantSignature string
-}
-
-// qualityFitGoldenVectors returns the known-answer vectors that pin quality
-// and fit as signed components. The three default-value vectors use the
-// effective quality (85) and fit ("cover") the handler applies when a URL
-// omits q and fit, so they are the signatures real signed URLs must carry.
-func qualityFitGoldenVectors() []qualityFitGoldenVector {
-	return []qualityFitGoldenVector{
-		{
-			name: "resized, default quality and fit",
-			req: signature.Request{
-				SourceHost: testHost,
-				SourcePath: testPath,
-				Width:      800,
-				Height:     600,
-				Format:     testFormatWebP,
-				Quality:    85,
-				FitMode:    qfFitCover,
-			},
-			// "cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200:85:cover"
-			wantSignature: "kdqeGoW2SX7qnaYtoB970wEnLydn0UnIgQYQLfAnjXQ=",
-		},
-		{
-			name: "resized with query, default quality and fit",
-			req: signature.Request{
-				SourceHost:  testHost,
-				SourcePath:  testPath,
-				SourceQuery: "token=abc&v=2",
-				Width:       800,
-				Height:      600,
-				Format:      testFormatWebP,
-				Quality:     85,
-				FitMode:     qfFitCover,
-			},
-			// "cdn.example.com:/photos/cat.jpg:token=abc&v=2:800:600:webp:1704067200:85:cover"
-			wantSignature: "pKgVBOTd_Q_EikI7MNQLC9Q8Hurdxzyv3EIYvVhqc2I=",
-		},
-		{
-			name: "original size, default quality and fit",
-			req: signature.Request{
-				SourceHost: testHost,
-				SourcePath: testPath,
-				Width:      0,
-				Height:     0,
-				Format:     testFormatPNG,
-				Quality:    85,
-				FitMode:    qfFitCover,
-			},
-			// "cdn.example.com:/photos/cat.jpg::0:0:png:1704067200:85:cover"
-			wantSignature: "6_rZ0yyVbGZRs8kG7n7HLgLi5Jt8vjiWQljIEL1jbIs=",
-		},
-		{
-			name: "non-default quality and fit",
-			req: signature.Request{
-				SourceHost: testHost,
-				SourcePath: testPath,
-				Width:      800,
-				Height:     600,
-				Format:     testFormatWebP,
-				Quality:    40,
-				FitMode:    qfFitContain,
-			},
-			// "cdn.example.com:/photos/cat.jpg::800:600:webp:1704067200:40:contain"
-			wantSignature: "pGaXpPUbI3A7nMx-4T9bfq9bYWBNL0kY4bxlcv3g1F8=",
-		},
-	}
-}
-
-// TestSigner_GoldenVectors_QualityFit pins the exact HMAC-SHA256 signature
-// output for requests that carry quality and fit as signed components. If
-// these assertions fail, the signed byte format
-// ("host:path:query:width:height:format:expiration:quality:fit") or the
-// base64url encoding has changed, breaking every signature already issued.
-// Update these constants only as part of a deliberate, documented signature
-// format migration.
-func TestSigner_GoldenVectors_QualityFit(t *testing.T) {
-	t.Parallel()
-
-	signer := signature.New(qfSigningKey)
-
-	for _, tt := range qualityFitGoldenVectors() {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			signReq := tt.req
-			signReq.Expires = time.Unix(qfExpiresUnix, 0)
-
-			gotSignature := signer.Sign(&signReq)
-			if gotSignature != tt.wantSignature {
-				t.Errorf("Sign() = %q, want %q (signed byte format changed?)",
-					gotSignature, tt.wantSignature)
-			}
-		})
-	}
-}

internal/signature/quality_fit_test.go

@@ -1,68 +0,0 @@
-package signature_test
-
-import (
-	"errors"
-	"testing"
-	"time"
-
-	"sneak.berlin/go/pixa/internal/signature"
-)
-
-// signedQualityFitRequest returns a request signed for quality 85 and fit
-// mode "cover", the effective defaults the handler applies before
-// verification.
-func signedQualityFitRequest(signer *signature.Signer) *signature.Request {
-	req := &signature.Request{
-		SourceHost: testHost,
-		SourcePath: testPath,
-		Width:      800,
-		Height:     600,
-		Format:     testFormatWebP,
-		Quality:    85,
-		FitMode:    "cover",
-		Expires:    time.Now().Add(1 * time.Hour),
-	}
-	req.Signature = signer.Sign(req)
-
-	return req
-}
-
-// TestSigner_Verify_QualityAndFitAreSigned proves that quality and fit are
-// covered by the signature: a URL signed for one quality or fit mode must
-// not verify when replayed with a different quality or fit mode. This is the
-// amplification vector from the issue — one signed URL replayed across many
-// quality and fit values yields many unauthorized cache entries and
-// transcodes — so it must be rejected.
-func TestSigner_Verify_QualityAndFitAreSigned(t *testing.T) {
-	t.Parallel()
-
-	signer := signature.New("test-secret-key")
-
-	cases := []struct {
-		name   string
-		tamper func(r *signature.Request)
-	}{
-		{
-			name:   "replayed with different quality",
-			tamper: func(r *signature.Request) { r.Quality = 40 },
-		},
-		{
-			name:   "replayed with different fit mode",
-			tamper: func(r *signature.Request) { r.FitMode = "contain" },
-		},
-	}
-
-	for _, tt := range cases {
-		t.Run(tt.name, func(t *testing.T) {
-			t.Parallel()
-
-			req := signedQualityFitRequest(signer)
-			tt.tamper(req)
-
-			err := signer.Verify(req)
-			if !errors.Is(err, signature.ErrInvalid) {
-				t.Errorf("Verify() = %v, want %v", err, signature.ErrInvalid)
-			}
-		})
-	}
-}

internal/signature/signature.go

@@ -37,15 +37,6 @@ type Request struct {
 	Height int
 	// Format is the requested output format (e.g. "webp").
 	Format string
-	// Quality is the requested output quality (1-100) for lossy formats.
-	// It is the effective value the request resolves to: callers pass the
-	// default quality when the request omits the parameter, so an omitted
-	// quality signs identically to that same value stated explicitly.
-	Quality int
-	// FitMode is how the image is fit into the requested dimensions
-	// (e.g. "cover"). Like Quality it is the effective value: callers pass
-	// the default fit mode when the request omits the parameter.
-	FitMode string
 	// Signature is the HMAC signature to verify.
 	Signature string
 	// Expires is the signature expiration timestamp.
@@ -65,8 +56,7 @@ func New(secretKey string) *Signer {
 }
 
 // Sign generates an HMAC-SHA256 signature for the given request.
-// The signature covers: host + path + query + width + height + format +
+// The signature covers: host + path + query + width + height + format + expiration.
-// expiration + quality + fit.
 func (s *Signer) Sign(req *Request) string {
 	data := s.buildSignatureData(req)
 	mac := hmac.New(sha256.New, s.secretKey)
@@ -78,8 +68,7 @@ func (s *Signer) Sign(req *Request) string {
 
 // Verify checks if the signature on the request is valid and not expired.
 // Signatures are exact-match only: every component of the signed data
-// (host, path, query, dimensions, format, expiration, quality, fit) must
+// (host, path, query, dimensions, format, expiration) must match exactly.
-// match exactly.
 // No suffix matching, wildcard matching, or partial matching is supported.
 // A signature for "cdn.example.com" will NOT verify for "example.com" or
 // "other.cdn.example.com", and vice versa.
@@ -153,13 +142,11 @@ func (s *Signer) GenerateSignedURL(
 }
 
 // buildSignatureData creates the string to be signed.
-// Format: "host:path:query:width:height:format:expiration:quality:fit"
+// Format: "host:path:query:width:height:format:expiration"
 // All components are used verbatim (exact match). No normalization,
-// suffix matching, or wildcard expansion is performed. Quality and fit
+// suffix matching, or wildcard expansion is performed.
-// are the effective transform values, so replaying a signed URL with a
-// different quality or fit mode fails verification.
 func (s *Signer) buildSignatureData(req *Request) string {
-	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d:%d:%s",
+	return fmt.Sprintf("%s:%s:%s:%d:%d:%s:%d",
 		req.SourceHost,
 		req.SourcePath,
 		req.SourceQuery,
@@ -167,8 +154,6 @@ func (s *Signer) buildSignatureData(req *Request) string {
 		req.Height,
 		req.Format,
 		req.Expires.Unix(),
-		req.Quality,
-		req.FitMode,
 	)
 }
 

internal/templates/generator.html

@@ -47,6 +47,7 @@
         {{end}}
 
         <form method="POST" action="/generate" class="bg-white rounded-lg shadow-md p-6 space-y-4">
+            {{ .CSRFField }}
             <div>
                 <label for="url" class="block text-sm font-medium text-gray-700 mb-1">
                     Source URL

internal/templates/login.html

@@ -17,6 +17,7 @@
         {{end}}
 
         <form method="POST" action="/" class="space-y-4">
+            {{ .CSRFField }}
             <div>
                 <label for="key" class="block text-sm font-medium text-gray-700 mb-1">
                     Signing Key

script/test

@@ -17,7 +17,11 @@ run_with_cgo_deps() {
 main() {
     cd "$ROOT"
     echo "Running tests..."
-    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -race -v ./..."
+    # Run without -v first for clean output on success; on failure rerun
+    # with -v for full diagnostics, then exit non-zero (REPO_POLICIES.md
+    # conditional-verbose-rerun pattern). The first run already proved the
+    # tests broken, so the build fails even if the rerun happens to pass.
+    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -race -cover ./... || { echo '--- Rerunning with -v for details ---'; CGO_ENABLED=1 go test -timeout 30s -race -v ./...; exit 1; }"
 }
 
 main "$@"