7 changed files with 350 additions and 3 deletions
@@ -100,6 +100,17 @@ than once, is refused with 400.
 - `<format>`: one of `orig`, `png`, `jpeg`, `webp`
 - `<size>`: `orig` or `<width>x<height>` (e.g. `800x600`)

+The login form (`POST /`) is limited to 5 attempts per minute per client
+address, counting an IPv6 client by its /64; an attempt over the limit is
+refused with 429 and a `Retry-After` header. Behind a reverse proxy the client
+address comes from `X-Forwarded-For` only when the proxy's address is in
+`trusted_proxies`; otherwise all users behind the proxy are counted as one
+client. With the default `trusted_proxies` (the RFC 1918 ranges), a client
+with a private address can choose the address it is counted by through its own
+`X-Forwarded-For`, whether it connects directly or through the proxy, because
+its own address is trusted too. Setting `trusted_proxies` to the proxy's own
+address closes this.
+
 ### Source Hosts

 Source hosts may be allowlisted in the configuration. Non-allowlisted
@@ -205,7 +216,8 @@ Key settings in more detail:
  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.
+  is ignored, so a client connecting directly from an address outside
+  these ranges cannot spoof its address.
  An omitted key defaults to the RFC 1918 private ranges (`10.0.0.0/8`,
  `172.16.0.0/12`, `192.168.0.0/16`), since pixa is deployed behind a
  proxy on a private network; an explicitly empty list (`[]`) trusts no
@@ -30,6 +30,15 @@ exhaustion

 # Completed Steps

+- 2026-09-28 rate limit the login form (closes #66): `POST /` is limited to 5
+  attempts per minute per client address, and an attempt over the limit is
+  refused with 429 and a `Retry-After` header; the address is the one
+  `internal/clientip` resolves through `trusted_proxies`, an IPv6 client is
+  counted by its /64, and an IPv4-mapped address as the IPv4 address it
+  carries; the limit is a `RateLimit` middleware in `internal/middleware` on
+  `github.com/go-chi/httprate`, which the image routes can reuse; the library
+  keeps counts for the current and the previous minute only; documented in
+  `README.md`.
 - 2026-09-28 refuse an unparseable `exp` on `/v1/image/` and log swallowed
  cache errors (closes #72): an `exp` in the URL that is not a whole
  number, an empty `exp=` included, is a 400 naming `exp` and the value,
@@ -245,7 +254,7 @@ exhaustion
 - P1: strip EXIF and other metadata from processed images (privacy)
 - P2: security
    - referer blacklist
-    - per-IP rate limiting
+    - per-IP rate limiting on the image routes
    - per-origin rate limiting
 - P2: HTTP response handling
    - Last-Modified headers
@@ -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/go-chi/httprate v0.16.0
 	github.com/gorilla/csrf v1.7.3
 	github.com/gorilla/securecookie v1.1.2
 	github.com/prometheus/client_golang v1.23.2
@@ -91,6 +92,7 @@ require (
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/josharian/intern v1.0.0 // indirect
 	github.com/json-iterator/go v1.1.12 // indirect
+	github.com/klauspost/cpuid/v2 v2.2.10 // indirect
 	github.com/kylelemons/godebug v1.1.0 // indirect
 	github.com/mailru/easyjson v0.7.7 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
@@ -113,6 +115,7 @@ require (
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.0 // indirect
 	github.com/x448/float16 v0.8.4 // indirect
+	github.com/zeebo/xxh3 v1.0.2 // indirect
 	go.etcd.io/etcd/api/v3 v3.6.2 // indirect
 	go.etcd.io/etcd/client/pkg/v3 v3.6.2 // indirect
 	go.etcd.io/etcd/client/v3 v3.6.2 // indirect
@@ -114,6 +114,8 @@ github.com/go-chi/chi/v5 v5.2.3 h1:WQIt9uxdsAbgIYgid+BpYc+liqQZGMHRaUwp0JUcvdE=
 github.com/go-chi/chi/v5 v5.2.3/go.mod h1:L2yAIGWB3H+phAw1NxKwWM+7eUH/lU8pOMm5hHcoops=
 github.com/go-chi/cors v1.2.2 h1:Jmey33TE+b+rB7fT8MUy1u0I4L+NARQlK6LhzKPSyQE=
 github.com/go-chi/cors v1.2.2/go.mod h1:sSbTewc+6wYHBBCW7ytsFSn836hqM7JxpglAy2Vzc58=
+github.com/go-chi/httprate v0.16.0 h1:8V5DH9j6pSK6UQoBsTpvMyFxycqaKEIToyPKzHJjUa8=
+github.com/go-chi/httprate v0.16.0/go.mod h1:A8lo+qRhk+s9LiuP5saS7XCGDXRXMcrueq0NfIuCa/I=
 github.com/go-errors/errors v1.4.2 h1:J6MZopCL4uSllY1OfXM374weqZFFItUbrImctkmUxIA=
 github.com/go-errors/errors v1.4.2/go.mod h1:sIVyrIiJhuEF+Pj9Ebtd6P/rEYROXFi3BopGUQ5a5Og=
 github.com/go-jose/go-jose/v4 v4.0.5 h1:M6T8+mKZl/+fNNuFHvGIzDz7BTLQPIounk/b9dw3AaE=
@@ -251,6 +253,8 @@ github.com/kisielk/errcheck v1.5.0/go.mod h1:pFxgyoBC7bSaBwPgfKdkLd5X25qrDl4LWUI
 github.com/kisielk/gotool v1.0.0/go.mod h1:XhKaO+MFFWcvkIS/tQcRk01m1F5IRFswLeQ+oQHNcck=
 github.com/klauspost/compress v1.18.0 h1:c/Cqfb0r+Yi+JtIEq73FWXVkRonBlf0CRNYc8Zttxdo=
 github.com/klauspost/compress v1.18.0/go.mod h1:2Pp+KzxcywXVXMr50+X0Q/Lsb43OQHYWRCY2AiWywWQ=
+github.com/klauspost/cpuid/v2 v2.2.10 h1:tBs3QSyvjDyFTq3uoc/9xFpCuOsJQFNPiAhYdw2skhE=
+github.com/klauspost/cpuid/v2 v2.2.10/go.mod h1:hqwkgyIinND0mEev00jJYCxPNVRVXFQeu1XKlok6oO0=
 github.com/konsorten/go-windows-terminal-sequences v1.0.1/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
 github.com/kr/logfmt v0.0.0-20140226030751-b84e30acd515/go.mod h1:+0opPa2QZZtGFBFZlji/RkVcI2GknAs/DXo4wKdlNEc=
 github.com/kr/pretty v0.1.0/go.mod h1:dAy3ld7l9f0ibDNOQOHHMYYIIbhfbHSm3C4ZsoJORNo=
@@ -396,6 +400,10 @@ github.com/x448/float16 v0.8.4/go.mod h1:14CWIYCyZA/cWjXOioeEpHeN/83MdbZDRQHoFcY
 github.com/yuin/goldmark v1.1.27/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+github.com/zeebo/assert v1.3.0 h1:g7C04CbJuIDKNPFHmsk4hwZDO5O+kntRxzaUoNXj+IQ=
+github.com/zeebo/assert v1.3.0/go.mod h1:Pq9JiuJQpG8JLJdtkwrJESF0Foym2/D9XMU5ciN/wJ0=
+github.com/zeebo/xxh3 v1.0.2 h1:xZmwmqxHZA8AI603jOQ0tMqmBr9lPeFwGg6d+xy9DC0=
+github.com/zeebo/xxh3 v1.0.2/go.mod h1:5NWz9Sef7zIDm2JHfFlcQvNekmcEl9ekUZQQKCYaDcA=
 go.etcd.io/etcd/api/v3 v3.6.2 h1:25aCkIMjUmiiOtnBIp6PhNj4KdcURuBak0hU2P1fgRc=
 go.etcd.io/etcd/api/v3 v3.6.2/go.mod h1:eFhhvfR8Px1P6SEuLT600v+vrhdDTdcfMzmnxVXXSbk=
 go.etcd.io/etcd/client/pkg/v3 v3.6.2 h1:zw+HRghi/G8fKpgKdOcEKpnBTE4OO39T6MegA0RopVU=
@@ -4,11 +4,13 @@ package middleware
 import (
 	"log/slog"
 	"net/http"
+	"net/netip"
 	"time"

 	basicauth "github.com/99designs/basicauth-go"
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/go-chi/cors"
+	"github.com/go-chi/httprate"
 	metrics "github.com/slok/go-http-metrics/metrics/prometheus"
 	ghmm "github.com/slok/go-http-metrics/middleware"
 	"github.com/slok/go-http-metrics/middleware/std"
@@ -88,6 +90,31 @@ func (s *Middleware) ClientIP() func(http.Handler) http.Handler {
 	}
 }

+// RateLimit returns a middleware that limits each client to requestLimit
+// requests per window and refuses a request over the limit with 429 Too Many
+// Requests and a Retry-After header. Clients are told apart by the address
+// the ClientIP middleware stored in the request context, so ClientIP must
+// run first. An IPv6 client is counted by its /64, which one client usually
+// holds whole; an IPv4-mapped address (::ffff:a.b.c.d) is counted as the
+// IPv4 address it carries, since every such address falls in the same /64.
+// Counts are kept only for the current and the previous window, so memory
+// stays bounded.
+func (s *Middleware) RateLimit(
+	requestLimit int, window time.Duration,
+) func(http.Handler) http.Handler {
+	return httprate.LimitBy(requestLimit, window,
+		func(r *http.Request) (string, error) {
+			ip := clientip.FromContext(r.Context())
+
+			addr, err := netip.ParseAddr(ip)
+			if err == nil {
+				ip = addr.Unmap().String()
+			}
+
+			return httprate.CanonicalizeIP(ip), nil
+		})
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter

@@ -0,0 +1,280 @@
+package server
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"net/url"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"testing"
+
+	"go.uber.org/fx/fxtest"
+
+	"sneak.berlin/go/pixa/internal/config"
+	"sneak.berlin/go/pixa/internal/database"
+	"sneak.berlin/go/pixa/internal/globals"
+	"sneak.berlin/go/pixa/internal/handlers"
+	"sneak.berlin/go/pixa/internal/logger"
+	"sneak.berlin/go/pixa/internal/middleware"
+)
+
+// testSigningKey is a throwaway signing key; submitting it logs in.
+const testSigningKey = "test-signing-key-0123456789abcdef"
+
+// wrongKey is submitted for a failed login.
+const wrongKey = "not-the-signing-key"
+
+// Addresses for the login rate limit tests. The test server trusts
+// 10.0.0.0/8 as its proxies, so the X-Forwarded-For sent by proxyPeer is
+// believed and the one sent by firstClient or secondClient is ignored.
+const (
+	firstClient     = "198.51.100.1:40000"
+	secondClient    = "198.51.100.2:40000"
+	proxyPeer       = "10.0.0.1:40000"
+	firstForwarded  = "203.0.113.1"
+	secondForwarded = "203.0.113.2"
+)
+
+// csrfFieldPattern extracts the CSRF token rendered into the login form.
+var csrfFieldPattern = regexp.MustCompile(
+	`name="gorilla\.csrf\.Token" value="([^"]+)"`)
+
+// newTestServer builds the server's real routes from the constructors
+// cmd/pixad uses, with a throwaway state directory. Debug marks requests
+// as plain HTTP, so the CSRF check runs without an https Referer.
+func newTestServer(t *testing.T) *Server {
+	t.Helper()
+
+	stateDir := t.TempDir()
+	cfg := &config.Config{
+		Debug:          true,
+		SigningKey:     testSigningKey,
+		StateDir:       stateDir,
+		DBURL:          "file:" + filepath.Join(stateDir, "state.sqlite3"),
+		TrustedProxies: []netip.Prefix{netip.MustParsePrefix("10.0.0.0/8")},
+	}
+
+	lc := fxtest.NewLifecycle(t)
+
+	log, err := logger.New(lc, logger.Params{Globals: &globals.Globals{}})
+	if err != nil {
+		t.Fatalf("logger.New() error = %v", err)
+	}
+
+	db, err := database.New(lc, database.Params{Logger: log, Config: cfg})
+	if err != nil {
+		t.Fatalf("database.New() error = %v", err)
+	}
+
+	h, err := handlers.New(lc, handlers.Params{
+		Logger: log, Database: db, Config: cfg,
+	})
+	if err != nil {
+		t.Fatalf("handlers.New() error = %v", err)
+	}
+
+	mw, err := middleware.New(lc, middleware.Params{Logger: log, Config: cfg})
+	if err != nil {
+		t.Fatalf("middleware.New() error = %v", err)
+	}
+
+	lc.RequireStart()
+	t.Cleanup(lc.RequireStop)
+
+	s := &Server{config: cfg, mw: mw, h: h}
+	s.SetupRoutes()
+
+	return s
+}
+
+// clientRequest builds a request for / arriving from remoteAddr, carrying
+// forwardedFor as its X-Forwarded-For header when that is not empty.
+func clientRequest(
+	t *testing.T, method string, body io.Reader, remoteAddr, forwardedFor string,
+) *http.Request {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), method, "/", body)
+	req.RemoteAddr = remoteAddr
+
+	if forwardedFor != "" {
+		req.Header.Set("X-Forwarded-For", forwardedFor)
+	}
+
+	return req
+}
+
+// postLogin loads the login form with GET / and submits key in it with
+// POST /, as a browser does, both from the same client. GET / is not rate
+// limited, so the form must load even for a client over the limit.
+func postLogin(
+	t *testing.T, s *Server, remoteAddr, forwardedFor, key string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	page := httptest.NewRecorder()
+	s.ServeHTTP(page,
+		clientRequest(t, http.MethodGet, nil, remoteAddr, forwardedFor))
+
+	if page.Code != http.StatusOK {
+		t.Fatalf("GET / status = %d, want %d", page.Code, http.StatusOK)
+	}
+
+	match := csrfFieldPattern.FindStringSubmatch(page.Body.String())
+	if match == nil {
+		t.Fatalf("no CSRF token field found in the login form")
+	}
+
+	form := url.Values{"key": {key}, "gorilla.csrf.Token": {match[1]}}
+	req := clientRequest(t, http.MethodPost,
+		strings.NewReader(form.Encode()), remoteAddr, forwardedFor)
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	for _, c := range page.Result().Cookies() {
+		req.AddCookie(c)
+	}
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// tripLoginRateLimit makes LoginAttemptsPerMinute failed logins from one
+// client, each answered with the login form again, then one more, which
+// must be refused with 429. It returns the response to that last attempt.
+func tripLoginRateLimit(
+	t *testing.T, s *Server, remoteAddr, forwardedFor string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	for attempt := range LoginAttemptsPerMinute {
+		rec := postLogin(t, s, remoteAddr, forwardedFor, wrongKey)
+		if rec.Code != http.StatusOK {
+			t.Fatalf("failed login %d status = %d, want %d",
+				attempt+1, rec.Code, http.StatusOK)
+		}
+	}
+
+	rec := postLogin(t, s, remoteAddr, forwardedFor, wrongKey)
+	if rec.Code != http.StatusTooManyRequests {
+		t.Fatalf("login over the limit status = %d, want %d",
+			rec.Code, http.StatusTooManyRequests)
+	}
+
+	return rec
+}
+
+// TestLoginRateLimitRefusesAttemptOverLimit verifies the login attempt
+// after LoginAttemptsPerMinute failed ones from one client is refused with
+// 429 and a Retry-After header, and that the client cannot get around the
+// limit by sending X-Forwarded-For: from a peer that is not a trusted
+// proxy, the header is ignored.
+func TestLoginRateLimitRefusesAttemptOverLimit(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	rec := tripLoginRateLimit(t, s, firstClient, "")
+
+	retryAfter := rec.Header().Get("Retry-After")
+
+	seconds, err := strconv.Atoi(retryAfter)
+	if err != nil || seconds <= 0 {
+		t.Errorf("Retry-After = %q, want a positive number of seconds",
+			retryAfter)
+	}
+
+	rec = postLogin(t, s, firstClient, secondForwarded, wrongKey)
+	if rec.Code != http.StatusTooManyRequests {
+		t.Errorf("login with X-Forwarded-For from an untrusted peer "+
+			"status = %d, want %d", rec.Code, http.StatusTooManyRequests)
+	}
+}
+
+// TestLoginRateLimitLeavesOtherClientsAlone verifies one client going over
+// the limit does not limit another: a failed login from a different
+// address is answered with the login form, and the signing key still logs
+// it in.
+func TestLoginRateLimitLeavesOtherClientsAlone(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	tripLoginRateLimit(t, s, firstClient, "")
+
+	rec := postLogin(t, s, secondClient, "", wrongKey)
+	if rec.Code != http.StatusOK {
+		t.Errorf("failed login from another client status = %d, want %d",
+			rec.Code, http.StatusOK)
+	}
+
+	rec = postLogin(t, s, secondClient, "", testSigningKey)
+	if rec.Code != http.StatusSeeOther {
+		t.Errorf("login with the signing key from another client "+
+			"status = %d, want %d", rec.Code, http.StatusSeeOther)
+	}
+}
+
+// TestLoginRateLimitCountsClientsBehindProxySeparately verifies the limit
+// counts the client address resolved from X-Forwarded-For, not the address
+// of the trusted proxy the requests arrive from, so two clients behind the
+// same proxy are counted separately.
+func TestLoginRateLimitCountsClientsBehindProxySeparately(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	tripLoginRateLimit(t, s, proxyPeer, firstForwarded)
+
+	rec := postLogin(t, s, proxyPeer, secondForwarded, wrongKey)
+	if rec.Code != http.StatusOK {
+		t.Errorf("failed login from a second client behind the proxy "+
+			"status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}
+
+// TestLoginRateLimitCountsIPv6ClientsByPrefix verifies an IPv6 client is
+// counted by its /64: another address in the same /64 is refused too,
+// while an address in a different /64 is not.
+func TestLoginRateLimitCountsIPv6ClientsByPrefix(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	tripLoginRateLimit(t, s, proxyPeer, "2001:db8::1")
+
+	rec := postLogin(t, s, proxyPeer, "2001:db8::2", wrongKey)
+	if rec.Code != http.StatusTooManyRequests {
+		t.Errorf("login from the same /64 status = %d, want %d",
+			rec.Code, http.StatusTooManyRequests)
+	}
+
+	rec = postLogin(t, s, proxyPeer, "2001:db8:0:1::1", wrongKey)
+	if rec.Code != http.StatusOK {
+		t.Errorf("login from another /64 status = %d, want %d",
+			rec.Code, http.StatusOK)
+	}
+}
+
+// TestLoginRateLimitCountsIPv4MappedClientsSeparately verifies an IPv4
+// client that the proxy forwards in IPv4-mapped IPv6 form (::ffff:a.b.c.d)
+// is counted by its IPv4 address, not by the /64 that every such address
+// shares, so two of them behind the proxy are counted separately.
+func TestLoginRateLimitCountsIPv4MappedClientsSeparately(t *testing.T) {
+	t.Parallel()
+
+	s := newTestServer(t)
+
+	tripLoginRateLimit(t, s, proxyPeer, "::ffff:"+firstForwarded)
+
+	rec := postLogin(t, s, proxyPeer, "::ffff:"+secondForwarded, wrongKey)
+	if rec.Code != http.StatusOK {
+		t.Errorf("failed login from a second IPv4-mapped client "+
+			"status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}
@@ -2,6 +2,7 @@ package server

 import (
 	"net/http"
+	"time"

 	sentryhttp "github.com/getsentry/sentry-go/http"
 	"github.com/go-chi/chi/v5"
@@ -12,6 +13,10 @@ import (
 	"sneak.berlin/go/pixa/internal/static"
 )

+// LoginAttemptsPerMinute is how many login attempts (POST /) one client may
+// make per minute; the next is refused with 429 Too Many Requests.
+const LoginAttemptsPerMinute = 5
+
 // SetupRoutes configures all HTTP routes.
 func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
@@ -50,11 +55,14 @@ func (s *Server) SetupRoutes() {
 	// token cookie is independent of the session cookie, so it also
 	// covers the login POST, where no session exists yet. LimitBody caps
 	// the POST body ahead of CSRF, which reads its token from that body.
+	// The login POST is rate limited per client after both, so every
+	// attempt that reaches the signing key comparison is counted.
 	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.With(s.mw.RateLimit(LoginAttemptsPerMinute, time.Minute)).
+			Post("/", s.h.HandleRoot())
 		r.Post("/generate", s.h.HandleGenerateURL())
 	})