build: run all linting in Docker via Dockerfile.lint (closes #104)

golangci-lint now runs only inside a container, never on the host.
script/lint builds a hash-pinned root Dockerfile.lint; the nix-shell and
host golangci-lint paths are gone. A per-run CACHEBUST build-arg is
folded into the lint step's cache key, so the linter re-executes on every
run and an unchanged tree cannot return a cached success having linted
nothing; script/lint fails a build that did not run the linter.

Dockerfile's lint stage runs golangci-lint directly, since make lint now
builds a container and there is no Docker inside a build. It is the same
image and config. golangci-lint config verify is left out: it fetches its
schema over an unpinned live HTTPS call, which REPO_POLICIES.md forbids.

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 ./...

TODO.md

@@ -1,21 +1,27 @@
 # 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
 
@@ -23,6 +29,15 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
+- 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; 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
 - 2026-08-07 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

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/handlers/auth.go

@@ -2,6 +2,7 @@ package handlers
 
 import (
 	"crypto/subtle"
+	"html/template"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -23,13 +24,13 @@ 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 +38,7 @@ 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
 	}
@@ -47,7 +48,7 @@ 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.renderLogin(w, "Invalid signing key")
+		s.renderLogin(w, r, "Invalid signing key")
 
 		return
 	}
@@ -56,7 +57,7 @@ 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
 	}
@@ -87,7 +88,7 @@ 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 +98,7 @@ 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 +109,7 @@ 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 +122,7 @@ 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 +187,20 @@ 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 +210,17 @@ 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 +229,9 @@ 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_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/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{

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/server/routes.go

@@ -44,11 +44,17 @@ 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.
+	s.router.Group(func(r chi.Router) {
+		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/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/lint

@@ -1,23 +1,55 @@
 #!/bin/sh
-# script/lint: run the linter. CGO dependencies (pkg-config, vips,
+# script/lint: run golangci-lint over the whole tree.
-# libheif) come from nix-shell when not already available (e.g. inside
+#
-# a Docker build or an existing nix-shell).
+# 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() {
+main() {
-    if command -v pkg-config >/dev/null 2>&1; then
+    cd "$ROOT"
-        sh -c "$1"
+
-    else
+    # A value no other run repeats. Dockerfile.lint folds it into the
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+    # lint step's cache key, so the linter re-executes every run instead
+    # of an unchanged tree returning a cached success having linted
+    # nothing.
+    cachebust="$(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

@@ -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 "$@"