build: stop installing golangci-lint on the host

The linter runs only inside the pinned golangci-lint container image
(Dockerfile.lint, and the Dockerfile lint stage). Remove the host
install that was still left behind: script/bootstrap's version and
sha256 pins, the release download, the ensure/verify helpers, and the
curl dependency they were the only user of; and drop golangci-lint from
the nix-shell package lists in the Makefile and script/test, which build
and test but never lint.

Model: opus-4-8

Dockerfile

@@ -13,9 +13,12 @@ RUN go mod download
 # Copy source code
 COPY . .
 
-# Run formatting check and linter
+# Run formatting check and linter. The linter is invoked directly, not
+# via `make lint`: `make lint` now builds Dockerfile.lint, and there is
+# no Docker inside a Docker build. This is the same linter, image, and
+# config that Dockerfile.lint and script/lint run.
 RUN make fmt-check
-RUN make lint
+RUN golangci-lint run --config .golangci.yml ./...
 
 # Build stage
 # golang:1.25.4-alpine, 2026-02-25

Dockerfile.lint

@@ -0,0 +1,41 @@
+# Dockerfile.lint: the one and only path that runs golangci-lint.
+#
+# golangci-lint is never installed on the host; it runs only inside this
+# build. A clean build of this file therefore IS a clean lint over the
+# whole tree. It runs the same linter and config as Dockerfile's lint
+# stage, pinned to the same image so the two cannot drift to different
+# linter versions.
+#
+# golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
+FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60
+
+# pixa is CGO/libvips: the type-aware linters compile every package, so
+# this image needs the same C libraries the build does.
+RUN apk add --no-cache build-base vips-dev libheif-dev pkgconfig
+
+WORKDIR /src
+
+# Modules first for layer caching; go.mod/go.sum settle this layer's
+# result, so it may safely be reused between runs.
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# Caching is deliberately waived for the lint step: an unchanged tree
+# must still run the linter, not return a cached success in well under a
+# second having linted nothing. CACHEBUST carries a value that differs
+# on every run (script/lint supplies it and refuses to build without
+# one). The lint RUN below references it, so BuildKit cannot serve that
+# step from cache. Keep the ${CACHEBUST} reference on that step: dropping
+# it lets the linter cache again and report a green that linted nothing.
+ARG CACHEBUST
+RUN test -n "${CACHEBUST}" || { \
+        echo "Dockerfile.lint requires the CACHEBUST build-arg; build it via script/lint." >&2; \
+        exit 1; }
+
+# `golangci-lint config verify` is deliberately not run: it fetches its
+# JSON schema over an unpinned live HTTPS call, which REPO_POLICIES.md
+# forbids for external references.
+RUN echo "pixa-lint: running golangci-lint (${CACHEBUST})" && \
+    golangci-lint run --config .golangci.yml ./...

Makefile

@@ -10,7 +10,7 @@ ifdef HAS_PKGCONFIG
   NIX_RUN_PREFIX =
   NIX_RUN_SUFFIX =
 else
-  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif golangci-lint git --run '
+  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif git --run '
   NIX_RUN_SUFFIX = '
 endif
 

README.md

@@ -24,7 +24,7 @@ $EDITOR config.yml   # replace the signing_key placeholder
 
 # or build and run via Docker
 make docker
-docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixad:latest
+docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
 ```
 
 A container is configured two ways. The signing key comes from the
@@ -122,6 +122,10 @@ 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
 - `upstream_fetch_timeout` — timeout for origin requests
 - `upstream_max_response_size` — max origin response size
 - `downstream_timeout` — client response timeout

TODO.md

@@ -25,10 +25,39 @@ The disk cache is now size-bounded with LRU eviction
 
 # 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 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 run all linting in Docker via `Dockerfile.lint` +
+  `script/lint` (closes #104): `script/lint` builds a hash-pinned root
+  `Dockerfile.lint`, and no host or nix-shell `golangci-lint` path
+  remains (`script/bootstrap` installs no linter, and the Makefile and
+  `script/test` nix-shell package lists carry only build/test deps); a
+  per-run `CACHEBUST` build-arg forces the lint step to execute every
+  run, so an unchanged tree cannot return a cached green that linted
+  nothing; `Dockerfile`'s lint stage runs `golangci-lint` directly,
+  since `make lint` now builds a container and there is no Docker inside
+  a build; `golangci-lint config verify` stays out, as it fetches its
+  schema over an unpinned live HTTPS call
+- 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
@@ -122,8 +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.example.yml

@@ -22,6 +22,15 @@ 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
+
 # Allow HTTP upstream (only for testing, always use HTTPS in production)
 allow_http: false
 

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"
@@ -42,6 +43,7 @@ const (
 	keyAllowHTTP                  = "allow_http"
 	keyUpstreamConnectionsPerHost = "upstream_connections_per_host"
 	keyCacheMaxBytes              = "cache_max_bytes"
+	keyBlockedNetworks            = "blocked_networks"
 )
 
 // placeholderSigningKey is the dummy signing_key shipped in
@@ -60,6 +62,7 @@ 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")
@@ -109,6 +112,11 @@ 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
+
 	// 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
@@ -177,6 +185,11 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		}
 	}
 
+	blockedNetworks, err := getBlockedNetworks(sc)
+	if err != nil {
+		return nil, err
+	}
+
 	loader := &strictLoader{sc: sc}
 
 	c := &Config{
@@ -192,7 +205,8 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		AllowHTTP:       loader.boolVal(keyAllowHTTP, false),
 		UpstreamConnectionsPerHost: loader.intVal(
 			keyUpstreamConnectionsPerHost, DefaultUpstreamConnectionsPerHost),
-		CacheMaxBytes: loader.int64Val(keyCacheMaxBytes, 0),
+		CacheMaxBytes:   loader.int64Val(keyCacheMaxBytes, 0),
+		BlockedNetworks: blockedNetworks,
 	}
 
 	// The computed default for cache_max_bytes needs a validated
@@ -224,7 +238,7 @@ func newFromSmartConfig(sc *smartconfig.Config) (*Config, error) {
 		return nil, loader.err
 	}
 
-	err := c.validate()
+	err = c.validate()
 	if err != nil {
 		return nil, err
 	}
@@ -308,7 +322,7 @@ func isKnownConfigKey(key string) bool {
 	switch key {
 	case keyDebug, keyMaintenanceMode, keyPort, keyStateDir, keySentryDSN,
 		keyDBURL, keyMetrics, keySigningKey, keyAllowlistHosts, keyAllowHTTP,
-		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, "env":
+		keyUpstreamConnectionsPerHost, keyCacheMaxBytes, keyBlockedNetworks, "env":
 		return true
 	}
 
@@ -802,3 +816,87 @@ func getStringSlice(sc *smartconfig.Config) []string {
 
 	return nil
 }
+
+// getBlockedNetworks parses the blocked_networks value 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; a default (the built-in
+// blocklist alone) applies only to an omitted key.
+func getBlockedNetworks(sc *smartconfig.Config) ([]netip.Prefix, error) {
+	if sc == nil {
+		return nil, nil
+	}
+
+	raw, ok := sc.Get(keyBlockedNetworks)
+	if !ok {
+		return nil, nil
+	}
+
+	if raw == nil {
+		return nil, errNullConfigValue(keyBlockedNetworks)
+	}
+
+	entries, err := blockedNetworkEntries(raw)
+	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",
+				keyBlockedNetworks, entry, errNotAValidCIDR)
+		}
+
+		prefixes = append(prefixes, prefix)
+	}
+
+	return prefixes, nil
+}
+
+// blockedNetworkEntries extracts the raw blocked_networks entries 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 blockedNetworkEntries(raw any) ([]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",
+					keyBlockedNetworks, item, item, errNotAString)
+			}
+
+			if strings.TrimSpace(str) == "" {
+				return nil, fmt.Errorf("config key %q: %w",
+					keyBlockedNetworks, 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",
+					keyBlockedNetworks, val, errEmptyEntry)
+			}
+
+			entries = append(entries, trimmed)
+		}
+
+		return entries, nil
+	default:
+		return nil, fmt.Errorf("config key %q: value %v (%T) is %w",
+			keyBlockedNetworks, raw, raw, errNotAStringList)
+	}
+}

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/handlers.go

@@ -111,6 +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/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
+	}
+
+	addr = addr.Unmap()
+
+	return slices.ContainsFunc(builtinBlockedPrefixes, func(prefix netip.Prefix) bool {
+		return prefix.Contains(addr)
+	})
 }
 
-// ssrfSafeDialer is a custom dialer that validates IP addresses before connecting.
+// 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
@@ -468,8 +532,10 @@ func ssrfSafeDialer(ctx context.Context, network, addr string) (net.Conn, error)
 	}
 
 	// Check all resolved IPs
-	if slices.ContainsFunc(ips, isPrivateIP) {
-		return nil, ErrSSRFBlocked
+	for _, ip := range ips {
+		if isBlockedIP(ip, blocked) {
+			return nil, ErrSSRFBlocked
+		}
 	}
 
 	// Connect using the first valid IP

internal/server/http.go

@@ -9,24 +9,40 @@ import (
 
 // HTTP server configuration constants.
 const (
-	HTTPReadTimeout    = 30 * time.Second
-	HTTPWriteTimeout   = 60 * time.Second
+	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() {
-	listenAddr := fmt.Sprintf(":%d", s.config.Port)
-	s.httpServer = &http.Server{
-		Addr:           listenAddr,
-		ReadTimeout:    HTTPReadTimeout,
-		WriteTimeout:   HTTPWriteTimeout,
-		MaxHeaderBytes: HTTPMaxHeaderBytes,
-		Handler:        s,
+// newHTTPServer builds the http.Server with the hardening timeouts and
+// limits applied. It is separate from serveUntilShutdown so the
+// configuration can be asserted in a test without binding a listener.
+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"
 )
 
@@ -46,8 +47,10 @@ func (s *Server) SetupRoutes() {
 
 	// Login/generator UI. The form routes carry CSRF protection; the
 	// token cookie is independent of the session cookie, so it also
-	// covers the login POST, where no session exists yet.
+	// 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())

script/bootstrap

@@ -3,20 +3,13 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go). golangci-lint is packaged in nix, brew, and apk; on apt
-# it is installed from a hash-verified GitHub release archive (never
-# curl | sh). CGO image libraries (pkg-config, vips, libheif) are
-# installed for the govips bindings.
+# make, or go). The linter is never installed on the host: golangci-lint
+# runs only inside Dockerfile.lint (see script/lint). CGO image libraries
+# (pkg-config, vips, libheif) are installed for the govips bindings.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-08-07. Never "latest"; exact versions only.
-GOLANGCI_LINT_VERSION="2.12.2"
-# sha256 of golangci-lint-2.12.2-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="8df580d2670fed8fa984aac0507099af8df275e665215f5c7a2ae3943893a553"
-GOLANGCI_LINT_SHA256_ARM64="44cd40a8c76c86755375adfeea52cfd3533cb43d7bd647771e0ae065e166df3a"
-
 PKGMGR=""
 SUDO=""
 
@@ -57,52 +50,6 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# verify_sha256 <file> <expected-hash>
-verify_sha256() {
-    if command -v sha256sum >/dev/null 2>&1; then
-        actual="$(sha256sum "$1" | cut -d' ' -f1)"
-    else
-        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
-    fi
-    if [ "$actual" != "$2" ]; then
-        echo "bootstrap: sha256 mismatch for $1" >&2
-        echo "  expected: $2" >&2
-        echo "  actual:   $actual" >&2
-        exit 1
-    fi
-}
-
-# apt has no golangci-lint package: install a pinned release archive
-# from GitHub, verified by hardcoded sha256 (never curl | sh).
-install_golangci_lint_release() {
-    case "$(uname -m)" in
-        x86_64) goarch="amd64"; sha="$GOLANGCI_LINT_SHA256_AMD64" ;;
-        aarch64|arm64) goarch="arm64"; sha="$GOLANGCI_LINT_SHA256_ARM64" ;;
-        *)
-            echo "bootstrap: unsupported architecture $(uname -m)" >&2
-            exit 1
-            ;;
-    esac
-    if missing curl; then pkg_install curl curl curl curl; fi
-    name="golangci-lint-${GOLANGCI_LINT_VERSION}-linux-${goarch}"
-    tmp="$(mktemp -d)"
-    curl -fsSL -o "$tmp/$name.tar.gz" \
-        "https://github.com/golangci/golangci-lint/releases/download/v${GOLANGCI_LINT_VERSION}/${name}.tar.gz"
-    verify_sha256 "$tmp/$name.tar.gz" "$sha"
-    tar -xzf "$tmp/$name.tar.gz" -C "$tmp"
-    $SUDO install -m 0755 "$tmp/$name/golangci-lint" /usr/local/bin/golangci-lint
-    rm -rf "$tmp"
-}
-
-ensure_golangci_lint() {
-    if ! missing golangci-lint; then return 0; fi
-    detect_pkgmgr
-    case "$PKGMGR" in
-        apt) install_golangci_lint_release ;;
-        *) pkg_install golangci-lint golangci-lint golangci-lint golangci-lint ;;
-    esac
-}
-
 # CGO dependencies for govips (image processing)
 ensure_cgo_deps() {
     if missing pkg-config; then
@@ -123,9 +70,8 @@ main() {
     if missing git; then pkg_install git git git git; fi
     if missing make; then pkg_install gnumake make make make; fi
 
-    # Go toolchain and linter
+    # Go toolchain
     if missing go; then pkg_install go golang go go; fi
-    ensure_golangci_lint
 
     # CGO image libraries
     ensure_cgo_deps

script/lint

@@ -1,23 +1,55 @@
 #!/bin/sh
-# script/lint: run the linter. CGO dependencies (pkg-config, vips,
-# libheif) come from nix-shell when not already available (e.g. inside
-# a Docker build or an existing nix-shell).
+# script/lint: run golangci-lint over the whole tree.
+#
+# The linter is never installed on the host: it runs only inside the
+# Dockerfile.lint build, one way, everywhere. A clean build is a clean
+# lint. See Dockerfile.lint for why the lint step cannot be cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-run_with_cgo_deps() {
-    if command -v pkg-config >/dev/null 2>&1; then
-        sh -c "$1"
-    else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+main() {
+    cd "$ROOT"
+
+    # A value no other run repeats. Dockerfile.lint folds it into the
+    # lint step's cache key, so the linter re-executes every run instead
+    # of an unchanged tree returning a cached success having linted
+    # nothing.
+    cachebust="$(date +%s)-$$"
+
+    tmp="$(mktemp -d "${TMPDIR:-/tmp}/pixa-lint.XXXXXX")"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+
+    # --progress=plain so the lint step's own output reaches the log we
+    # check below; --output=type=cacheonly because we want the linter's
+    # verdict, not an image left in the local store. The build status
+    # travels through a file: a pipeline's exit status is tee's, not the
+    # build's.
+    (
+        set +e
+        docker build \
+            --progress=plain \
+            --build-arg CACHEBUST="$cachebust" \
+            --output=type=cacheonly \
+            -f Dockerfile.lint . 2>&1
+        echo "$?" >"$tmp/status"
+    ) | tee "$tmp/build.log"
+
+    status="$(cat "$tmp/status" 2>/dev/null || echo 1)"
+    [ "${status:-1}" -eq 0 ] || exit "${status:-1}"
+
+    # The linter's start line must appear as build output, not only in
+    # the build's echo of the RUN instruction. A step served from cache
+    # prints the instruction and none of its output; a step that runs
+    # prints a "#<n> <elapsed> ..." output line. Requiring that output
+    # line means a future edit dropping the CACHEBUST reference from
+    # Dockerfile.lint fails here rather than passing having linted
+    # nothing.
+    if ! grep -Eq '^#[0-9]+ +[0-9]+\.[0-9]+ +pixa-lint: running golangci-lint' \
+            "$tmp/build.log"; then
+        echo "script/lint: golangci-lint did not execute (cached step?)." >&2
+        exit 1
     fi
 }
 
-main() {
-    cd "$ROOT"
-    echo "Running linter..."
-    run_with_cgo_deps "golangci-lint run"
-}
-
 main "$@"

script/test

@@ -10,7 +10,7 @@ run_with_cgo_deps() {
     if command -v pkg-config >/dev/null 2>&1; then
         sh -c "$1"
     else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+        nix-shell -p pkg-config vips libheif git --run "$1"
     fi
 }