diff --git a/backend/cmd/web/main.go b/backend/cmd/web/main.go
index 9424bb4..f81f6bf 100644
--- a/backend/cmd/web/main.go
+++ b/backend/cmd/web/main.go
@@ -43,6 +43,16 @@ func main() {
 		os.Exit(1)
 	}
 
+	// Load the CSRF authentication key from SESSION_SECRET env var (D-15).
+	// Fails fast with a clear message if missing or wrong length — the server
+	// cannot operate without a valid CSRF key (AUTH-06).
+	csrfKey, err := auth.LoadKeyFromEnv()
+	if err != nil {
+		slog.Error("invalid SESSION_SECRET", "err", err,
+			"hint", "generate with: openssl rand -hex 32")
+		os.Exit(1)
+	}
+
 	// signal.NotifyContext (Go 1.21+) is the canonical idiom — equivalent
 	// to signal.Notify + a channel but the resulting ctx propagates the
 	// cancellation through to handlers, pgxpool dialing, etc.
@@ -67,7 +77,7 @@ func main() {
 
 	deps := web.AuthDeps{Queries: q, Store: store, Secure: secure, Limiter: rl}
 
-	router := web.NewRouter(pool, "./static", deps)
+	router := web.NewRouter(pool, "./static", deps, csrfKey, env)
 
 	srv := &http.Server{
 		Addr:    ":" + port,
diff --git a/backend/go.mod b/backend/go.mod
index a9dee34..edd8d02 100644
--- a/backend/go.mod
+++ b/backend/go.mod
@@ -6,13 +6,14 @@ require (
 	github.com/a-h/templ v0.3.1020
 	github.com/go-chi/chi/v5 v5.2.5
 	github.com/google/uuid v1.6.0
+	github.com/gorilla/csrf v1.7.3
 	github.com/jackc/pgx/v5 v5.9.2
 	github.com/pressly/goose/v3 v3.27.1
 	golang.org/x/crypto v0.51.0
+	golang.org/x/time v0.15.0
 )
 
 require (
-	github.com/gorilla/csrf v1.7.3 // indirect
 	github.com/gorilla/securecookie v1.1.2 // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
@@ -23,5 +24,4 @@ require (
 	golang.org/x/sync v0.20.0 // indirect
 	golang.org/x/sys v0.44.0 // indirect
 	golang.org/x/text v0.37.0 // indirect
-	golang.org/x/time v0.15.0 // indirect
 )
diff --git a/backend/go.sum b/backend/go.sum
index a89f98d..b5da560 100644
--- a/backend/go.sum
+++ b/backend/go.sum
@@ -9,6 +9,8 @@ github.com/go-chi/chi/v5 v5.2.5 h1:Eg4myHZBjyvJmAFjFvWgrqDTXFyOzjj7YIm3L3mu6Ug=
 github.com/go-chi/chi/v5 v5.2.5/go.mod h1:X7Gx4mteadT3eDOMTsXzmI4/rwUpOwBHLpAfupzFJP0=
 github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
 github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+github.com/google/gofuzz v1.2.0 h1:xRy4A+RhZaiKjJ1bPfwQ8sedCA+YS2YcCHW6ec7JMi0=
+github.com/google/gofuzz v1.2.0/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
 github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
 github.com/gorilla/csrf v1.7.3 h1:BHWt6FTLZAb2HtWT5KDBf6qgpZzvtbp9QWDRKZMXJC0=
diff --git a/backend/internal/auth/csrf.go b/backend/internal/auth/csrf.go
new file mode 100644
index 0000000..fd973b2
--- /dev/null
+++ b/backend/internal/auth/csrf.go
@@ -0,0 +1,83 @@
+package auth
+
+import (
+	"encoding/hex"
+	"errors"
+	"net/http"
+	"os"
+
+	"github.com/gorilla/csrf"
+)
+
+// ErrCSRFKeyInvalid is returned by LoadKeyFromEnv when SESSION_SECRET is
+// missing or decodes to a length other than 32 bytes.
+var ErrCSRFKeyInvalid = errors.New("SESSION_SECRET must be a 64-char hex string encoding 32 bytes; generate with: openssl rand -hex 32")
+
+// LoadKeyFromEnv reads SESSION_SECRET from the environment, hex-decodes it,
+// and validates that the result is exactly 32 bytes. Returns ErrCSRFKeyInvalid
+// for a missing, non-hex, or wrong-length value. The caller (cmd/web/main.go)
+// should log.Fatalf on error.
+func LoadKeyFromEnv() ([]byte, error) {
+	raw := os.Getenv("SESSION_SECRET")
+	if raw == "" {
+		return nil, ErrCSRFKeyInvalid
+	}
+	key, err := hex.DecodeString(raw)
+	if err != nil {
+		return nil, ErrCSRFKeyInvalid
+	}
+	if len(key) != 32 {
+		return nil, ErrCSRFKeyInvalid
+	}
+	return key, nil
+}
+
+// Mount returns a gorilla/csrf middleware configured with the locked options
+// from CONTEXT D-14 / D-24:
+//
+//   - csrf.Secure(env != "dev"): sets the Secure flag on the _gorilla_csrf
+//     cookie in all environments except "dev" (plain-HTTP local development).
+//   - csrf.SameSite(csrf.SameSiteLaxMode): SameSite=Lax interim defense.
+//   - csrf.Path("/"): the CSRF cookie is scoped to the entire site.
+//   - csrf.FieldName("_csrf"): the hidden form field name (matches @ui.CSRFField).
+//   - csrf.RequestHeader("X-CSRF-Token"): accepted header for HTMX hx-headers usage.
+//
+// The middleware is mounted AFTER auth.ResolveSession and BEFORE any route
+// group (D-24, Pitfall 7).
+//
+// When env == "dev", requests are additionally marked as plaintext HTTP via
+// csrf.PlaintextHTTPRequest so gorilla/csrf skips the Referer-based origin
+// check (which only applies to TLS). This allows local development and
+// integration tests running over plain HTTP to function correctly.
+//
+// trustedOrigins is an optional list of additional trusted origins (used in
+// tests to allow localhost requests without a Referer header).
+// In production, pass nil — SameSite=Lax and the CSRF cookie handle the defense.
+func Mount(env string, key []byte, trustedOrigins ...string) func(http.Handler) http.Handler {
+	opts := []csrf.Option{
+		csrf.Secure(env != "dev"),
+		csrf.SameSite(csrf.SameSiteLaxMode),
+		csrf.Path("/"),
+		csrf.FieldName("_csrf"),
+		csrf.RequestHeader("X-CSRF-Token"),
+	}
+	if len(trustedOrigins) > 0 {
+		opts = append(opts, csrf.TrustedOrigins(trustedOrigins))
+	}
+	csrfMiddleware := csrf.Protect(key, opts...)
+
+	// In dev mode, mark every request as plaintext HTTP so gorilla/csrf skips
+	// the Referer-based TLS origin check. This is safe: dev mode already has
+	// Secure=false on the cookie, and SameSite=Lax provides interim protection.
+	if env == "dev" {
+		return func(next http.Handler) http.Handler {
+			return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				// Tag the request as plaintext HTTP before csrf.Protect sees it.
+				r = csrf.PlaintextHTTPRequest(r)
+				csrfMiddleware(next).ServeHTTP(w, r)
+			})
+		}
+	}
+
+	return csrfMiddleware
+}
diff --git a/backend/internal/web/csrf_debug_test.go b/backend/internal/web/csrf_debug_test.go
new file mode 100644
index 0000000..ab10d49
--- /dev/null
+++ b/backend/internal/web/csrf_debug_test.go
@@ -0,0 +1,66 @@
+package web
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"backend/internal/auth"
+	"backend/internal/db/sqlc"
+)
+
+func TestCSRF_Debug(t *testing.T) {
+	pool, cleanup := setupTestDB(t)
+	defer cleanup()
+
+	q := sqlc.New(pool)
+	store := auth.NewStore(q)
+	router := newTestRouter(q, store)
+
+	// GET /login and collect cookies
+	getReq := httptest.NewRequest(http.MethodGet, "/login", nil)
+	getRec := httptest.NewRecorder()
+	router.ServeHTTP(getRec, getReq)
+
+	t.Logf("GET /login status: %d", getRec.Code)
+	t.Logf("GET /login cookies:")
+	for _, c := range getRec.Result().Cookies() {
+		t.Logf("  %s=%s (httponly=%v, secure=%v)", c.Name, c.Value[:min(len(c.Value), 20)], c.HttpOnly, c.Secure)
+	}
+
+	body := getRec.Body.String()
+	const needle = `name="_csrf" value="`
+	idx := strings.Index(body, needle)
+	if idx == -1 {
+		t.Fatal("no _csrf hidden input found")
+	}
+	rest := body[idx+len(needle):]
+	end := strings.Index(rest, `"`)
+	token := rest[:end]
+	t.Logf("Extracted CSRF token: %s...", token[:min(len(token), 20)])
+
+	form := url.Values{"email": {"x@y.com"}, "password": {"correct-horse-12"}, "_csrf": {token}}
+	postReq := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
+	postReq.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range getRec.Result().Cookies() {
+		t.Logf("Adding cookie to POST: %s", c.Name)
+		postReq.AddCookie(c)
+	}
+	t.Logf("POST cookies count: %d", len(getRec.Result().Cookies()))
+	
+	postRec := httptest.NewRecorder()
+	router.ServeHTTP(postRec, postReq)
+	t.Logf("POST /login status: %d", postRec.Code)
+	if postRec.Code == 403 {
+		t.Logf("403 body: %s", postRec.Body.String()[:min(len(postRec.Body.String()), 200)])
+	}
+}
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
diff --git a/backend/internal/web/csrf_test.go b/backend/internal/web/csrf_test.go
index 36e1ad5..ed6da18 100644
--- a/backend/internal/web/csrf_test.go
+++ b/backend/internal/web/csrf_test.go
@@ -14,13 +14,15 @@ import (
 )
 
 // newTestRouterWithCSRF builds a router with CSRF enabled using a test key.
+// "localhost" is added as a trusted origin so httptest requests without a
+// Referer header are accepted.
 func newTestRouterWithCSRF(q *sqlc.Queries, store *auth.Store) http.Handler {
 	csrfKey := make([]byte, 32)
 	for i := range csrfKey {
 		csrfKey[i] = byte(i + 1)
 	}
 	deps := AuthDeps{Queries: q, Store: store, Secure: false}
-	return NewRouter(stubPinger{}, "./static", deps, csrfKey, "dev")
+	return NewRouter(stubPinger{}, "./static", deps, csrfKey, "dev", "localhost")
 }
 
 // extractCSRFToken performs a GET request and extracts the _csrf token from the
diff --git a/backend/internal/web/handlers.go b/backend/internal/web/handlers.go
index 936f8f1..500acdd 100644
--- a/backend/internal/web/handlers.go
+++ b/backend/internal/web/handlers.go
@@ -8,6 +8,8 @@ import (
 
 	"backend/internal/auth"
 	"backend/templates"
+
+	"github.com/gorilla/csrf"
 )
 
 // HealthzHandler returns an HTTP handler that probes the supplied Pinger
@@ -38,12 +40,14 @@ func HealthzHandler(pinger Pinger) http.HandlerFunc {
 // IndexHandler renders the root page (templates.Index) as text/html.
 // The authenticated user is pulled from the request context (set by
 // auth.ResolveSession) and passed to the template so the layout header can
-// render the logout button and the page can show the user's email.
+// render the logout button and email.
+// csrf.Token(r) is threaded into the template so the logout form includes the
+// hidden _csrf field (AUTH-06, D-14).
 func IndexHandler() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		_, user, _ := auth.Authed(r.Context())
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		_ = templates.Index(user).Render(r.Context(), w)
+		_ = templates.Index(user, csrf.Token(r)).Render(r.Context(), w)
 	}
 }
 
diff --git a/backend/internal/web/handlers_auth.go b/backend/internal/web/handlers_auth.go
index ffd90e8..1d3eef6 100644
--- a/backend/internal/web/handlers_auth.go
+++ b/backend/internal/web/handlers_auth.go
@@ -12,6 +12,7 @@ import (
 	"backend/internal/db/sqlc"
 	"backend/templates"
 
+	"github.com/gorilla/csrf"
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgconn"
 )
@@ -49,7 +50,7 @@ func clientIP(r *http.Request) string {
 func SignupPageHandler() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		_ = templates.SignupPage(templates.SignupForm{}, templates.SignupErrors{}).Render(r.Context(), w)
+		_ = templates.SignupPage(templates.SignupForm{}, templates.SignupErrors{}, csrf.Token(r)).Render(r.Context(), w)
 	}
 }
 
@@ -61,11 +62,13 @@ func SignupPageHandler() http.HandlerFunc {
 //   - Email is not logged on validation errors (T-2-18).
 //   - Duplicate email is detected via pgconn error code 23505 (T-2-19).
 //   - A fresh session token is created on every signup (T-2-04).
+//   - Form values are read via r.PostFormValue ONLY (never r.Body) so gorilla/csrf
+//     body consumption does not interfere (Pitfall 1, T-2-08c).
 func SignupPostHandler(deps AuthDeps) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		ctx := r.Context()
 
-		// 1. Read form values.
+		// 1. Read form values via r.PostFormValue (Pitfall 1: never read r.Body directly).
 		email := strings.TrimSpace(r.PostFormValue("email"))
 		password := r.PostFormValue("password")
 
@@ -147,9 +150,9 @@ func renderSignupError(w http.ResponseWriter, r *http.Request, form templates.Si
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	w.WriteHeader(status)
 	if r.Header.Get("HX-Request") == "true" {
-		_ = templates.SignupFormFragment(form, errs).Render(r.Context(), w)
+		_ = templates.SignupFormFragment(form, errs, csrf.Token(r)).Render(r.Context(), w)
 	} else {
-		_ = templates.SignupPage(form, errs).Render(r.Context(), w)
+		_ = templates.SignupPage(form, errs, csrf.Token(r)).Render(r.Context(), w)
 	}
 }
 
@@ -157,7 +160,7 @@ func renderSignupError(w http.ResponseWriter, r *http.Request, form templates.Si
 func LoginPageHandler() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "text/html; charset=utf-8")
-		_ = templates.LoginPage(templates.LoginForm{}, templates.LoginErrors{}).Render(r.Context(), w)
+		_ = templates.LoginPage(templates.LoginForm{}, templates.LoginErrors{}, csrf.Token(r)).Render(r.Context(), w)
 	}
 }
 
@@ -176,11 +179,13 @@ func LoginPageHandler() http.HandlerFunc {
 //     wrong password to prevent user enumeration (D-20, T-2-03).
 //   - Password is never logged (T-2-21).
 //   - Session rotated on every successful login (T-2-04, D-10).
+//   - Form values are read via r.PostFormValue ONLY (never r.Body) so gorilla/csrf
+//     body consumption does not interfere (Pitfall 1, T-2-08c).
 func LoginPostHandler(deps AuthDeps) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		ctx := r.Context()
 
-		// 1. Read form values.
+		// 1. Read form values via r.PostFormValue (Pitfall 1: never read r.Body directly).
 		email := strings.TrimSpace(r.PostFormValue("email"))
 		password := r.PostFormValue("password")
 
@@ -271,9 +276,9 @@ func renderLoginError(w http.ResponseWriter, r *http.Request, form templates.Log
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	w.WriteHeader(status)
 	if r.Header.Get("HX-Request") == "true" {
-		_ = templates.LoginFormFragment(form, errs).Render(r.Context(), w)
+		_ = templates.LoginFormFragment(form, errs, csrf.Token(r)).Render(r.Context(), w)
 	} else {
-		_ = templates.LoginPage(form, errs).Render(r.Context(), w)
+		_ = templates.LoginPage(form, errs, csrf.Token(r)).Render(r.Context(), w)
 	}
 }
 
@@ -286,6 +291,7 @@ func renderLoginError(w http.ResponseWriter, r *http.Request, form templates.Log
 //   - Store.Delete hard-deletes the session row (D-06, T-2-07).
 //   - ClearSessionCookie sets Max-Age=-1 to expire the browser cookie (D-06).
 //   - HTMX requests receive 200 + HX-Redirect; plain requests receive 303 (D-22).
+//   - CSRF token validated by gorilla/csrf middleware before this handler runs (AUTH-06).
 func LogoutHandler(deps AuthDeps) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		// Defense-in-depth: RequireAuth already gates this route, but guard here
diff --git a/backend/internal/web/handlers_auth_test.go b/backend/internal/web/handlers_auth_test.go
index cd9a5ae..054ef55 100644
--- a/backend/internal/web/handlers_auth_test.go
+++ b/backend/internal/web/handlers_auth_test.go
@@ -17,17 +17,62 @@ import (
 	"backend/internal/db/sqlc"
 )
 
+// testCSRFKey is a fixed 32-byte key used by all test routers. It is NOT a
+// real key — safe to have here because it is test-only and dev-env scoped.
+var testCSRFKey = func() []byte {
+	key := make([]byte, 32)
+	for i := range key {
+		key[i] = byte(i + 1)
+	}
+	return key
+}()
+
 // newTestRouter builds a router backed by a real DB for integration tests.
+// CSRF is enabled with a fixed test key and env="dev" (Secure=false on cookie).
+// "localhost" is added as a trusted origin so httptest requests without a
+// Referer header are accepted.
 func newTestRouter(q *sqlc.Queries, store *auth.Store) http.Handler {
 	deps := AuthDeps{Queries: q, Store: store, Secure: false}
-	return NewRouter(stubPinger{}, "./static", deps)
+	return NewRouter(stubPinger{}, "./static", deps, testCSRFKey, "dev", "localhost")
 }
 
 // newTestRouterWithLimiter builds a router with an injected LimiterStore,
 // enabling rate-limit tests to use a fake clock.
 func newTestRouterWithLimiter(q *sqlc.Queries, store *auth.Store, rl *auth.LimiterStore) http.Handler {
 	deps := AuthDeps{Queries: q, Store: store, Secure: false, Limiter: rl}
-	return NewRouter(stubPinger{}, "./static", deps)
+	return NewRouter(stubPinger{}, "./static", deps, testCSRFKey, "dev", "localhost")
+}
+
+// getCSRFToken performs a GET request to path and extracts the CSRF token
+// from the rendered form HTML. Returns the token string and any Set-Cookie
+// headers (including the gorilla_csrf cookie) from the response.
+func getCSRFToken(t *testing.T, router http.Handler, path string, cookies []*http.Cookie) (token string, respCookies []*http.Cookie) {
+	t.Helper()
+	req := httptest.NewRequest(http.MethodGet, path, nil)
+	for _, c := range cookies {
+		req.AddCookie(c)
+	}
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+
+	body := rec.Body.String()
+	const needle = `name="_csrf" value="`
+	idx := strings.Index(body, needle)
+	if idx == -1 {
+		t.Fatalf("getCSRFToken: _csrf hidden input not found in GET %s; body snippet: %.300s", path, body)
+	}
+	rest := body[idx+len(needle):]
+	end := strings.Index(rest, `"`)
+	if end == -1 {
+		t.Fatalf("getCSRFToken: closing quote not found for _csrf in GET %s", path)
+	}
+	token = rest[:end]
+
+	respCookies = append(respCookies, cookies...)
+	for _, c := range rec.Result().Cookies() {
+		respCookies = append(respCookies, c)
+	}
+	return token, respCookies
 }
 
 // preInsertUser inserts a user with TestParams-hashed password directly via sqlc
@@ -81,9 +126,13 @@ func TestSignup_Success(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"alice@example.com"}, "password": {"correct-horse-12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"alice@example.com"}, "password": {"correct-horse-12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -138,10 +187,14 @@ func TestSignup_Success_HTMX(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"bob@example.com"}, "password": {"correct-horse-12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"bob@example.com"}, "password": {"correct-horse-12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("HX-Request", "true")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -162,9 +215,13 @@ func TestSignup_InvalidEmail(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"not-an-email"}, "password": {"correct-horse-12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"not-an-email"}, "password": {"correct-horse-12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -195,9 +252,13 @@ func TestSignup_PasswordTooShort(t *testing.T) {
 	router := newTestRouter(q, store)
 
 	// 11 chars — below the 12-char minimum.
-	form := url.Values{"email": {"carol@example.com"}, "password": {"short12345!"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"carol@example.com"}, "password": {"short12345!"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -227,9 +288,13 @@ func TestSignup_PasswordTooLong(t *testing.T) {
 	router := newTestRouter(q, store)
 
 	longPw := strings.Repeat("a", 129) // 129 chars — above the 128-char maximum.
-	form := url.Values{"email": {"dave@example.com"}, "password": {longPw}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"dave@example.com"}, "password": {longPw}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -262,9 +327,13 @@ func TestSignup_DuplicateEmail(t *testing.T) {
 	// Pre-insert a user using TestParams to avoid the slow DefaultParams hash.
 	preInsertUser(t, ctx, q, "eve@example.com", "correct-horse-12")
 
-	form := url.Values{"email": {"eve@example.com"}, "password": {"correct-horse-12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"eve@example.com"}, "password": {"correct-horse-12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -298,9 +367,13 @@ func TestSignup_EmailNormalized(t *testing.T) {
 	router := newTestRouter(q, store)
 
 	// Uppercase + whitespace email — must be stored trimmed and lowercased.
-	form := url.Values{"email": {"  Frank@Example.COM  "}, "password": {"correct-horse-12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/signup", nil)
+	form := url.Values{"email": {"  Frank@Example.COM  "}, "password": {"correct-horse-12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/signup", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -367,9 +440,13 @@ func TestLogin_Success(t *testing.T) {
 
 	user := preInsertUser(t, ctx, q, "test@example.com", "correct-horse-12chars")
 
-	form := url.Values{"email": {"test@example.com"}, "password": {"correct-horse-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"test@example.com"}, "password": {"correct-horse-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -406,10 +483,14 @@ func TestLogin_Success_HTMX(t *testing.T) {
 
 	preInsertUser(t, ctx, q, "test2@example.com", "correct-horse-12chars")
 
-	form := url.Values{"email": {"test2@example.com"}, "password": {"correct-horse-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"test2@example.com"}, "password": {"correct-horse-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("HX-Request", "true")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -433,9 +514,13 @@ func TestLogin_WrongPassword(t *testing.T) {
 
 	preInsertUser(t, ctx, q, "testpw@example.com", "correct-horse-12chars")
 
-	form := url.Values{"email": {"testpw@example.com"}, "password": {"wrong-password-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"testpw@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -464,9 +549,13 @@ func TestLogin_UnknownEmail(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"nouser@example.com"}, "password": {"correct-horse-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"nouser@example.com"}, "password": {"correct-horse-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -488,9 +577,13 @@ func TestLogin_ValidationError_BadEmail(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"not-an-email"}, "password": {"correct-horse-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"not-an-email"}, "password": {"correct-horse-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -511,9 +604,13 @@ func TestLogin_ValidationError_ShortPassword(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	form := url.Values{"email": {"testval@example.com"}, "password": {"shortpw12"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"testval@example.com"}, "password": {"shortpw12"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -542,10 +639,14 @@ func TestLogin_RotatesExistingSession(t *testing.T) {
 		t.Fatalf("store.Create: %v", err)
 	}
 
-	form := url.Values{"email": {"rotatetest@example.com"}, "password": {"correct-horse-12chars"}}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"rotatetest@example.com"}, "password": {"correct-horse-12chars"}, "_csrf": {csrfToken}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.AddCookie(&http.Cookie{Name: auth.SessionCookieName, Value: oldCookieValue})
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -625,11 +726,15 @@ func TestLogin_RateLimit_6thAttemptReturns429(t *testing.T) {
 	preInsertUser(t, ctx, q, "ratelimit@example.com", "correct-horse-12chars")
 
 	for i := 1; i <= 6; i++ {
-		form := url.Values{"email": {"ratelimit@example.com"}, "password": {"wrong-password-12chars"}}
+		csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+		form := url.Values{"email": {"ratelimit@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfToken}}
 		req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 		// Set RemoteAddr to a known IP so chimw.RealIP won't change it.
 		req.RemoteAddr = "192.168.1.1:12345"
+		for _, c := range csrfCookies {
+			req.AddCookie(c)
+		}
 		rec := httptest.NewRecorder()
 
 		router.ServeHTTP(rec, req)
@@ -666,11 +771,15 @@ func TestLogin_RateLimit_6thAttemptHTMXNoFullPage(t *testing.T) {
 	preInsertUser(t, ctx, q, "ratelimithtmx@example.com", "correct-horse-12chars")
 
 	for i := 1; i <= 6; i++ {
-		form := url.Values{"email": {"ratelimithtmx@example.com"}, "password": {"wrong-password-12chars"}}
+		csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+		form := url.Values{"email": {"ratelimithtmx@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfToken}}
 		req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 		req.Header.Set("HX-Request", "true")
 		req.RemoteAddr = "192.168.1.2:12345"
+		for _, c := range csrfCookies {
+			req.AddCookie(c)
+		}
 		rec := httptest.NewRecorder()
 
 		router.ServeHTTP(rec, req)
@@ -704,19 +813,27 @@ func TestLogin_RateLimit_KeyedByEmailPlusIP(t *testing.T) {
 
 	// Exhaust emailA from IP1.
 	for i := 0; i < 6; i++ {
-		form := url.Values{"email": {"emailA@example.com"}, "password": {"wrong-password-12chars"}}
+		csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+		form := url.Values{"email": {"emailA@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfToken}}
 		req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 		req.RemoteAddr = "10.0.0.1:1234"
+		for _, c := range csrfCookies {
+			req.AddCookie(c)
+		}
 		rec := httptest.NewRecorder()
 		router.ServeHTTP(rec, req)
 	}
 
 	// emailB from same IP1 should still be allowed (separate key).
-	form := url.Values{"email": {"emailB@example.com"}, "password": {"wrong-password-12chars"}}
+	csrfTokenB, csrfCookiesB := getCSRFToken(t, router, "/login", nil)
+	form := url.Values{"email": {"emailB@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfTokenB}}
 	req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.RemoteAddr = "10.0.0.1:1234"
+	for _, c := range csrfCookiesB {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 	router.ServeHTTP(rec, req)
 
@@ -738,10 +855,14 @@ func TestLogin_RateLimit_AppliesBeforeUserLookup(t *testing.T) {
 
 	// Use an email that does NOT exist in the DB.
 	for i := 0; i < 6; i++ {
-		form := url.Values{"email": {"nonexistent@example.com"}, "password": {"wrong-password-12chars"}}
+		csrfToken, csrfCookies := getCSRFToken(t, router, "/login", nil)
+		form := url.Values{"email": {"nonexistent@example.com"}, "password": {"wrong-password-12chars"}, "_csrf": {csrfToken}}
 		req := httptest.NewRequest(http.MethodPost, "/login", strings.NewReader(form.Encode()))
 		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 		req.RemoteAddr = "10.0.0.2:1234"
+		for _, c := range csrfCookies {
+			req.AddCookie(c)
+		}
 		rec := httptest.NewRecorder()
 		router.ServeHTTP(rec, req)
 
@@ -772,8 +893,13 @@ func TestLogout_Success(t *testing.T) {
 	}
 	sessionID := hashCookieValue(t, cookieValue)
 
-	req := httptest.NewRequest(http.MethodPost, "/logout", nil)
-	req.AddCookie(&http.Cookie{Name: auth.SessionCookieName, Value: cookieValue})
+	sessionCookie := &http.Cookie{Name: auth.SessionCookieName, Value: cookieValue}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/", []*http.Cookie{sessionCookie})
+	req := httptest.NewRequest(http.MethodPost, "/logout", strings.NewReader(url.Values{"_csrf": {csrfToken}}.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -819,18 +945,19 @@ func TestLogout_UnauthRedirectsToLogin(t *testing.T) {
 	store := auth.NewStore(q)
 	router := newTestRouter(q, store)
 
-	// POST /logout with NO cookie — RequireAuth must block it.
+	// POST /logout with NO cookie and NO CSRF token.
+	// gorilla/csrf runs before RequireAuth, so a missing CSRF token yields 403
+	// before RequireAuth can redirect to /login. Both 403 and 303 are acceptable
+	// here — the important invariant is that the request is rejected (not 500)
+	// and no logout side-effect occurs.
 	req := httptest.NewRequest(http.MethodPost, "/logout", nil)
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
 
-	// Must redirect to /login (from RequireAuth), NOT a 500.
-	if rec.Code != http.StatusSeeOther {
-		t.Fatalf("status = %d; want 303 (RequireAuth redirect)", rec.Code)
-	}
-	if loc := rec.Header().Get("Location"); loc != "/login" {
-		t.Errorf("Location = %q; want /login", loc)
+	// 403 (csrf rejected) or 303 (RequireAuth redirected) — both are correct.
+	if rec.Code != http.StatusForbidden && rec.Code != http.StatusSeeOther {
+		t.Fatalf("status = %d; want 403 (CSRF) or 303 (RequireAuth)", rec.Code)
 	}
 }
 
@@ -849,9 +976,14 @@ func TestLogout_HXRedirect(t *testing.T) {
 		t.Fatalf("store.Create: %v", err)
 	}
 
-	req := httptest.NewRequest(http.MethodPost, "/logout", nil)
-	req.AddCookie(&http.Cookie{Name: auth.SessionCookieName, Value: cookieValue})
+	sessionCookie := &http.Cookie{Name: auth.SessionCookieName, Value: cookieValue}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/", []*http.Cookie{sessionCookie})
+	req := httptest.NewRequest(http.MethodPost, "/logout", strings.NewReader(url.Values{"_csrf": {csrfToken}}.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	req.Header.Set("HX-Request", "true")
+	for _, c := range csrfCookies {
+		req.AddCookie(c)
+	}
 	rec := httptest.NewRecorder()
 
 	router.ServeHTTP(rec, req)
@@ -879,9 +1011,14 @@ func TestLogout_AfterLogoutSubsequentRequestUnauth(t *testing.T) {
 		t.Fatalf("store.Create: %v", err)
 	}
 
-	// Logout first.
-	logoutReq := httptest.NewRequest(http.MethodPost, "/logout", nil)
-	logoutReq.AddCookie(&http.Cookie{Name: auth.SessionCookieName, Value: cookieValue})
+	// Logout first — need to get a CSRF token from the protected page.
+	sessionCookie := &http.Cookie{Name: auth.SessionCookieName, Value: cookieValue}
+	csrfToken, csrfCookies := getCSRFToken(t, router, "/", []*http.Cookie{sessionCookie})
+	logoutReq := httptest.NewRequest(http.MethodPost, "/logout", strings.NewReader(url.Values{"_csrf": {csrfToken}}.Encode()))
+	logoutReq.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	for _, c := range csrfCookies {
+		logoutReq.AddCookie(c)
+	}
 	logoutRec := httptest.NewRecorder()
 	router.ServeHTTP(logoutRec, logoutReq)
 
diff --git a/backend/internal/web/handlers_test.go b/backend/internal/web/handlers_test.go
index 6f409ad..cf22580 100644
--- a/backend/internal/web/handlers_test.go
+++ b/backend/internal/web/handlers_test.go
@@ -66,7 +66,7 @@ func TestHealthz_Down(t *testing.T) {
 // was public. The HTMX demo content is tested by
 // TestProtected_HomeAuthRendersUserEmail in handlers_auth_test.go.
 func TestIndex_UnauthRedirects(t *testing.T) {
-	router := NewRouter(stubPinger{}, "./static", AuthDeps{})
+	router := NewRouter(stubPinger{}, "./static", AuthDeps{}, testCSRFKey, "dev")
 	rec := httptest.NewRecorder()
 	req := httptest.NewRequest(http.MethodGet, "/", nil)
 
@@ -81,7 +81,7 @@ func TestIndex_UnauthRedirects(t *testing.T) {
 }
 
 func TestDemoTime_Fragment(t *testing.T) {
-	router := NewRouter(stubPinger{}, "./static", AuthDeps{})
+	router := NewRouter(stubPinger{}, "./static", AuthDeps{}, testCSRFKey, "dev")
 	rec := httptest.NewRecorder()
 	req := httptest.NewRequest(http.MethodGet, "/demo/time", nil)
 
@@ -104,7 +104,7 @@ func TestDemoTime_Fragment(t *testing.T) {
 }
 
 func TestRequestID_HeaderSet(t *testing.T) {
-	router := NewRouter(stubPinger{}, "./static", AuthDeps{})
+	router := NewRouter(stubPinger{}, "./static", AuthDeps{}, testCSRFKey, "dev")
 	rec := httptest.NewRecorder()
 	req := httptest.NewRequest(http.MethodGet, "/healthz", nil)
 
diff --git a/backend/internal/web/router.go b/backend/internal/web/router.go
index cc677cb..c259402 100644
--- a/backend/internal/web/router.go
+++ b/backend/internal/web/router.go
@@ -26,8 +26,8 @@ type Pinger interface {
 //  2. chi RealIP
 //  3. SlogLoggerMiddleware (REPLACES chi's middleware.Logger — Pitfall 6)
 //  4. chi Recoverer (after Logger so panics carry request_id)
-//  5. auth.ResolveSession (reads session cookie, attaches user to context)
-//     NOTE: csrf.Protect is added in Plan 07.
+//  5. auth.ResolveSession (reads session cookie, attaches user to context) — D-24
+//  6. auth.Mount (gorilla/csrf — MUST come after ResolveSession, before routes) — D-24, Pitfall 7
 //
 // Routes: GET / · GET /healthz · GET /demo/time · GET /static/*
 //         GET /signup (auth pages, behind RedirectIfAuthed) · POST /signup.
@@ -36,13 +36,24 @@ type Pinger interface {
 //
 // deps.Store may be nil during unit tests for Phase 1 routes (those routes
 // never exercise session resolution). ResolveSession guards against nil Store.
-func NewRouter(pinger Pinger, staticDir string, deps AuthDeps) http.Handler {
+//
+// csrfKey is the 32-byte CSRF authentication key loaded from SESSION_SECRET.
+// env is the runtime environment string (e.g. "dev", "development", "production").
+// When env == "dev", the CSRF cookie Secure flag is disabled for plain-HTTP
+// local development (D-15, D-24).
+// trustedOrigins is an optional list of additional origins for the CSRF
+// referer check (used in integration tests to allow localhost requests without
+// a Referer header). In production, pass no extra args — leave empty.
+func NewRouter(pinger Pinger, staticDir string, deps AuthDeps, csrfKey []byte, env string, trustedOrigins ...string) http.Handler {
 	r := chi.NewRouter()
 	r.Use(RequestIDMiddleware)
 	r.Use(chimw.RealIP)
 	r.Use(SlogLoggerMiddleware(slog.Default()))
 	r.Use(chimw.Recoverer)
+	// D-24 locked order: ResolveSession BEFORE csrf.Protect (auth.Mount).
 	r.Use(auth.ResolveSession(deps.Store))
+	// D-24: gorilla/csrf runs after ResolveSession and before all route groups (Pitfall 7).
+	r.Use(auth.Mount(env, csrfKey, trustedOrigins...))
 
 	// Auth pages — redirect to / if already authenticated.
 	r.Group(func(r chi.Router) {
diff --git a/backend/internal/web/ui/csrf_field.templ b/backend/internal/web/ui/csrf_field.templ
new file mode 100644
index 0000000..d18e62d
--- /dev/null
+++ b/backend/internal/web/ui/csrf_field.templ
@@ -0,0 +1,9 @@
+package ui
+
+// CSRFField renders the hidden CSRF token input required by gorilla/csrf.
+// Every <form method="POST"> must include @ui.CSRFField(csrfToken) as its
+// first child so that the middleware can validate the double-submit cookie
+// (D-15, AUTH-06).
+templ CSRFField(token string) {
+	<input type="hidden" name="_csrf" value={ token }/>
+}
diff --git a/backend/templates/auth_login.templ b/backend/templates/auth_login.templ
index fb9ad40..139170c 100644
--- a/backend/templates/auth_login.templ
+++ b/backend/templates/auth_login.templ
@@ -5,13 +5,13 @@ import "backend/internal/web/ui"
 // LoginPage renders the full /login page wrapped in the base Layout.
 // It delegates the form section to LoginFormFragment so HTMX can swap just the
 // form on validation errors without re-rendering the surrounding shell.
-templ LoginPage(form LoginForm, errs LoginErrors) {
-	@Layout("Sign in", nil) {
+templ LoginPage(form LoginForm, errs LoginErrors, csrfToken string) {
+	@Layout("Sign in", nil, csrfToken) {
 		<div class="flex min-h-[60vh] items-start justify-center pt-16">
 			@ui.Card(nil) {
 				<div class="w-full max-w-sm px-6 py-8">
 					<h1 class="mb-6 text-2xl font-semibold">Sign in to your account</h1>
-					@LoginFormFragment(form, errs)
+					@LoginFormFragment(form, errs, csrfToken)
 				</div>
 			}
 		</div>
@@ -22,7 +22,7 @@ templ LoginPage(form LoginForm, errs LoginErrors) {
 // hx-post targets this component itself so the form can be replaced inline
 // on validation failure (D-19, D-20).
 // The outer id="login-form" must match the hx-target on this element.
-templ LoginFormFragment(form LoginForm, errs LoginErrors) {
+templ LoginFormFragment(form LoginForm, errs LoginErrors, csrfToken string) {
 	<form
 		id="login-form"
 		method="POST"
@@ -32,7 +32,7 @@ templ LoginFormFragment(form LoginForm, errs LoginErrors) {
 		hx-swap="outerHTML"
 		class="space-y-5"
 	>
-		<!-- CSRF field added in Plan 07 -->
+		@ui.CSRFField(csrfToken)
 		@GeneralError(errs.General)
 		<div>
 			<label for="email" class="block text-sm font-medium text-slate-700">Email address</label>
diff --git a/backend/templates/auth_signup.templ b/backend/templates/auth_signup.templ
index b7140f0..6acd258 100644
--- a/backend/templates/auth_signup.templ
+++ b/backend/templates/auth_signup.templ
@@ -5,13 +5,13 @@ import "backend/internal/web/ui"
 // SignupPage renders the full /signup page wrapped in the base Layout.
 // It delegates the form section to SignupFormFragment so HTMX can swap just the
 // form on validation errors without re-rendering the surrounding shell.
-templ SignupPage(form SignupForm, errs SignupErrors) {
-	@Layout("Sign up", nil) {
+templ SignupPage(form SignupForm, errs SignupErrors, csrfToken string) {
+	@Layout("Sign up", nil, csrfToken) {
 		<div class="flex min-h-[60vh] items-start justify-center pt-16">
 			@ui.Card(nil) {
 				<div class="w-full max-w-sm px-6 py-8">
 					<h1 class="mb-6 text-2xl font-semibold">Create your account</h1>
-					@SignupFormFragment(form, errs)
+					@SignupFormFragment(form, errs, csrfToken)
 				</div>
 			}
 		</div>
@@ -22,7 +22,7 @@ templ SignupPage(form SignupForm, errs SignupErrors) {
 // hx-post targets this component itself so the form can be replaced inline
 // on validation failure (D-19, D-25).
 // The outer id="signup-form" must match the hx-target on this element.
-templ SignupFormFragment(form SignupForm, errs SignupErrors) {
+templ SignupFormFragment(form SignupForm, errs SignupErrors, csrfToken string) {
 	<form
 		id="signup-form"
 		method="POST"
@@ -32,7 +32,7 @@ templ SignupFormFragment(form SignupForm, errs SignupErrors) {
 		hx-swap="outerHTML"
 		class="space-y-5"
 	>
-		<!-- CSRF field added in Plan 07 -->
+		@ui.CSRFField(csrfToken)
 		@GeneralError(errs.General)
 		<div>
 			<label for="email" class="block text-sm font-medium text-slate-700">Email address</label>
diff --git a/backend/templates/auth_signup_test.go b/backend/templates/auth_signup_test.go
index 3afb332..05a240f 100644
--- a/backend/templates/auth_signup_test.go
+++ b/backend/templates/auth_signup_test.go
@@ -11,7 +11,7 @@ import (
 // expected form attributes and that email value round-trips correctly.
 func TestSignupPage_RendersForm(t *testing.T) {
 	var buf bytes.Buffer
-	err := SignupPage(SignupForm{Email: "x@y.z"}, SignupErrors{}).Render(context.Background(), &buf)
+	err := SignupPage(SignupForm{Email: "x@y.z"}, SignupErrors{}, "testtoken").Render(context.Background(), &buf)
 	if err != nil {
 		t.Fatalf("SignupPage.Render: %v", err)
 	}
@@ -23,6 +23,7 @@ func TestSignupPage_RendersForm(t *testing.T) {
 		`action="/signup"`,
 		`hx-post="/signup"`,
 		`value="x@y.z"`,
+		`name="_csrf"`,
 	} {
 		if !strings.Contains(body, want) {
 			t.Errorf("SignupPage body missing %q", want)
@@ -36,7 +37,7 @@ func TestSignupPage_RendersForm(t *testing.T) {
 func TestSignupFormFragment_RendersErrors(t *testing.T) {
 	var buf bytes.Buffer
 	errs := SignupErrors{Password: "Password must be 12-128 characters"}
-	err := SignupFormFragment(SignupForm{}, errs).Render(context.Background(), &buf)
+	err := SignupFormFragment(SignupForm{}, errs, "testtoken").Render(context.Background(), &buf)
 	if err != nil {
 		t.Fatalf("SignupFormFragment.Render: %v", err)
 	}
@@ -55,7 +56,7 @@ func TestSignupFormFragment_RendersErrors(t *testing.T) {
 // (security requirement T-2-01, D-25).
 func TestSignupPage_DoesNotEchoPassword(t *testing.T) {
 	var buf bytes.Buffer
-	err := SignupPage(SignupForm{Email: "a@b.com", Password: "hunter2hunter2"}, SignupErrors{}).Render(context.Background(), &buf)
+	err := SignupPage(SignupForm{Email: "a@b.com", Password: "hunter2hunter2"}, SignupErrors{}, "testtoken").Render(context.Background(), &buf)
 	if err != nil {
 		t.Fatalf("SignupPage.Render: %v", err)
 	}
diff --git a/backend/templates/index.templ b/backend/templates/index.templ
index ba7ab14..535ead9 100644
--- a/backend/templates/index.templ
+++ b/backend/templates/index.templ
@@ -8,8 +8,10 @@ import (
 // Index renders the root page (protected, requires auth).
 // The user parameter is the authenticated user from request context, passed
 // through to Layout so the header can render the logout button and email.
-templ Index(user *auth.User) {
-	@Layout("Xtablo", user) {
+// csrfToken is passed to Layout so the logout form can include the hidden
+// _csrf field (AUTH-06, D-14).
+templ Index(user *auth.User, csrfToken string) {
+	@Layout("Xtablo", user, csrfToken) {
 		<p class="text-sm text-slate-500 mb-6">Signed in as { user.Email }</p>
 		<h1 class="text-[28px] font-semibold leading-tight">Xtablo</h1>
 		<p class="mt-2 text-base text-slate-600">
diff --git a/backend/templates/layout.templ b/backend/templates/layout.templ
index b4f5c28..1238314 100644
--- a/backend/templates/layout.templ
+++ b/backend/templates/layout.templ
@@ -3,7 +3,10 @@
 // generate`; generated files are gitignored.
 package templates
 
-import "backend/internal/auth"
+import (
+	"backend/internal/auth"
+	"backend/internal/web/ui"
+)
 
 // Layout is the base HTML shell every page renders inside. The structural
 // classes, container width (max-w-5xl), horizontal padding, header strip,
@@ -15,7 +18,10 @@ import "backend/internal/auth"
 // When non-nil, the header renders a Log out POST form (D-22). Auth pages
 // pass nil since they're gated behind RedirectIfAuthed and never shown to
 // authed users.
-templ Layout(title string, user *auth.User) {
+//
+// csrfToken is threaded from the handler via csrf.Token(r) so the logout
+// form can embed @ui.CSRFField(csrfToken) (AUTH-06, D-14).
+templ Layout(title string, user *auth.User, csrfToken string) {
 	<!DOCTYPE html>
 	<html lang="en">
 		<head>
@@ -32,7 +38,7 @@ templ Layout(title string, user *auth.User) {
 						<div class="flex items-center gap-3">
 							<span class="text-sm text-slate-600">{ user.Email }</span>
 							<form method="POST" action="/logout" class="inline">
-								<!-- CSRF field added in Plan 07 -->
+								@ui.CSRFField(csrfToken)
 								<button type="submit" class="text-sm text-slate-700 hover:underline">Log out</button>
 							</form>
 						</div>
diff --git a/backend/templates/layout_test.go b/backend/templates/layout_test.go
index 2389a02..ebdf266 100644
--- a/backend/templates/layout_test.go
+++ b/backend/templates/layout_test.go
@@ -11,10 +11,11 @@ import (
 
 // TestLayout_LogoutFormVisibleWhenAuthed verifies that the logout form is
 // rendered in the header when Layout receives a non-nil user (D-22).
+// The _csrf hidden field must also be present (AUTH-06).
 func TestLayout_LogoutFormVisibleWhenAuthed(t *testing.T) {
 	var buf bytes.Buffer
 	user := &auth.User{Email: "a@b.c"}
-	err := Layout("Test", user).Render(context.Background(), &buf)
+	err := Layout("Test", user, "mytesttoken").Render(context.Background(), &buf)
 	if err != nil {
 		t.Fatalf("Layout.Render: %v", err)
 	}
@@ -26,13 +27,19 @@ func TestLayout_LogoutFormVisibleWhenAuthed(t *testing.T) {
 	if !strings.Contains(body, `method="POST"`) {
 		t.Errorf("Layout body missing method=\"POST\"; logout must be a POST form (D-22)")
 	}
+	if !strings.Contains(body, `name="_csrf"`) {
+		t.Errorf("Layout body missing name=\"_csrf\"; logout form must embed CSRF field (AUTH-06)")
+	}
+	if !strings.Contains(body, `value="mytesttoken"`) {
+		t.Errorf("Layout body missing value=\"mytesttoken\"; CSRF token not threaded into form")
+	}
 }
 
 // TestLayout_LogoutFormHiddenWhenUnauthed verifies that no logout form is
 // rendered when Layout receives a nil user (unauthenticated request).
 func TestLayout_LogoutFormHiddenWhenUnauthed(t *testing.T) {
 	var buf bytes.Buffer
-	err := Layout("Test", nil).Render(context.Background(), &buf)
+	err := Layout("Test", nil, "").Render(context.Background(), &buf)
 	if err != nil {
 		t.Fatalf("Layout.Render: %v", err)
 	}