diff --git a/SPEC.md b/SPEC.md
index 9c95dc1..67df7b2 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -37,9 +37,12 @@ editing, or general reporting beyond the Tally view.
     email is present.
 - **Authorization gate:** the user MUST be a member of `REQUIRED_GROUP` (exact,
   case-sensitive match). Otherwise the callback returns **403**.
-- On success a `hsa_session` cookie is set for **12 hours**. Cookies are
-  `HttpOnly`, `SameSite=Lax`, and `Secure` whenever `REDIRECT_URL` is `https://`.
-  The session is an AES-encrypted token; the key is `SHA-256(SESSION_SECRET)`.
+- On success a `hsa_session` cookie is set for **12 hours**. The session is an
+  **AES-256-GCM** sealed token (tamper-evident; key = `SHA-256(SESSION_SECRET)`),
+  and the 12h lifetime is enforced **server-side** (a session older than the TTL is
+  rejected even if the cookie value is intact), not just via the cookie's MaxAge.
+  Cookies are `HttpOnly`, `SameSite=Lax`, and `Secure` whenever `REDIRECT_URL` is
+  `https://`.
 - `GET /logout` clears the session and redirects to `/login`.
 - **Public routes** (no session required): `/healthz`, `/static/*`, `/login`,
   `/callback`, `/logout`. **Every other route requires a valid session**; missing
@@ -203,7 +206,9 @@ filename, attachment count, and tags, with links to add another / Manage / Expor
 
 - `GET /receipt/{id}/file` and `GET /attachment/{id}/file` serve the stored bytes
   **from the DB blob** (so serving works even if the on-disk copy is gone), with
-  the original MIME type and `Content-Disposition: inline`.
+  the original MIME type, `Content-Disposition: inline`, and
+  `X-Content-Type-Options: nosniff` (so the browser won't sniff user bytes past the
+  declared, upload-time-allowlisted type).
 
 ---
 
diff --git a/internal/web/ai_test.go b/internal/web/ai_test.go
index d9790f9..bb375ef 100644
--- a/internal/web/ai_test.go
+++ b/internal/web/ai_test.go
@@ -7,6 +7,9 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
+
+	"maisym.com/hsa/internal/auth"
 )
 
 // uploadWithClassifyBlob posts a receipt plus a round-tripped AI suggestion blob.
@@ -85,6 +88,25 @@ func TestAI_NoMissWhenSuggestionMatches(t *testing.T) {
 	}
 }
 
+func TestAuth_StaleSessionRedirects(t *testing.T) {
+	s := testServerWithStore(t)
+	// A session issued well beyond the TTL must be rejected server-side even though
+	// the cookie value itself is a valid (untampered) sealed token.
+	old := auth.Session{Subject: "jm@example.com", Groups: []string{"hsa-users"},
+		IssuedAt: time.Now().Add(-13 * time.Hour)}
+	v, err := auth.EncodeSession(old, s.cfg.SessionKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	req := httptest.NewRequest(http.MethodGet, "/", nil)
+	req.AddCookie(&http.Cookie{Name: sessionCookie, Value: v})
+	rec := httptest.NewRecorder()
+	s.Routes().ServeHTTP(rec, req)
+	if rec.Code != http.StatusFound || rec.Header().Get("Location") != "/login" {
+		t.Errorf("stale session: status=%d location=%q, want 302 -> /login", rec.Code, rec.Header().Get("Location"))
+	}
+}
+
 func TestAI_NotesCRUD(t *testing.T) {
 	s := testServerWithStore(t)
 
diff --git a/internal/web/middleware.go b/internal/web/middleware.go
index feb6faa..b12aff1 100644
--- a/internal/web/middleware.go
+++ b/internal/web/middleware.go
@@ -27,6 +27,13 @@ func (s *Server) requireAuth(next http.Handler) http.Handler {
 			http.Redirect(w, r, "/login", http.StatusFound)
 			return
 		}
+		// Server-side expiry: a cookie's MaxAge is client-enforced, so also reject a
+		// session older than its TTL — a captured cookie value can't be replayed forever.
+		if time.Since(sess.IssuedAt) > sessionTTL {
+			s.clearCookie(w, sessionCookie)
+			http.Redirect(w, r, "/login", http.StatusFound)
+			return
+		}
 		ctx := context.WithValue(r.Context(), sessionKey, sess)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})
diff --git a/internal/web/upload_test.go b/internal/web/upload_test.go
index bff55ab..4c445e2 100644
--- a/internal/web/upload_test.go
+++ b/internal/web/upload_test.go
@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 	"testing"
+	"time"
 
 	"maisym.com/hsa/internal/auth"
 	"maisym.com/hsa/internal/config"
@@ -39,7 +40,7 @@ func testServerWithStore(t *testing.T) *Server {
 
 func authCookie(t *testing.T, s *Server) *http.Cookie {
 	t.Helper()
-	sess := auth.Session{Subject: "jm@example.com", Groups: []string{"hsa-users"}}
+	sess := auth.Session{Subject: "jm@example.com", Groups: []string{"hsa-users"}, IssuedAt: time.Now()}
 	v, err := auth.EncodeSession(sess, s.cfg.SessionKey)
 	if err != nil {
 		t.Fatal(err)
diff --git a/internal/web/views.go b/internal/web/views.go
index 7be742a..57a395b 100644
--- a/internal/web/views.go
+++ b/internal/web/views.go
@@ -215,6 +215,7 @@ func (s *Server) handleReceiptFile(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	w.Header().Set("Content-Type", rec.MimeType)
+	w.Header().Set("X-Content-Type-Options", "nosniff") // don't let the browser sniff user bytes
 	w.Header().Set("Content-Disposition", fmt.Sprintf("inline; filename=%q", rec.OriginalFilename))
 	http.ServeContent(w, r, rec.OriginalFilename, rec.UploadedAt, bytes.NewReader(rec.ImageData))
 }
@@ -228,6 +229,7 @@ func (s *Server) handleAttachmentFile(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	w.Header().Set("Content-Type", a.MimeType)
+	w.Header().Set("X-Content-Type-Options", "nosniff") // don't let the browser sniff user bytes
 	w.Header().Set("Content-Disposition", fmt.Sprintf("inline; filename=%q", a.OriginalFilename))
 	http.ServeContent(w, r, a.OriginalFilename, a.UploadedAt, bytes.NewReader(a.ImageData))
 }
diff --git a/internal/web/web.go b/internal/web/web.go
index b1a4a9c..833efff 100644
--- a/internal/web/web.go
+++ b/internal/web/web.go
@@ -19,6 +19,10 @@ import (
 const (
 	sessionCookie = "hsa_session"
 	loginCookie   = "hsa_login"
+
+	// sessionTTL bounds a session both as the cookie's MaxAge and as a server-side
+	// age check (see requireAuth), so a leaked cookie value can't be replayed forever.
+	sessionTTL = 12 * time.Hour
 )
 
 // Server holds the wired dependencies for the HTTP handlers.
@@ -224,7 +228,7 @@ func (s *Server) handleCallback(w http.ResponseWriter, r *http.Request) {
 		s.serverError(w, "encode session", err)
 		return
 	}
-	s.setCookie(w, sessionCookie, encoded, 12*time.Hour)
+	s.setCookie(w, sessionCookie, encoded, sessionTTL)
 	http.Redirect(w, r, "/", http.StatusFound)
 }
 
diff --git a/internal/web/web_test.go b/internal/web/web_test.go
index e972600..13ef0bd 100644
--- a/internal/web/web_test.go
+++ b/internal/web/web_test.go
@@ -5,6 +5,7 @@ import (
 	"net/http/httptest"
 	"strings"
 	"testing"
+	"time"
 
 	"maisym.com/hsa/internal/auth"
 	"maisym.com/hsa/internal/config"
@@ -64,7 +65,7 @@ func TestHome_NoSession_RedirectsToLogin(t *testing.T) {
 
 func TestHome_ValidSession_ShowsUploadForm(t *testing.T) {
 	s := testServerWithStore(t)
-	sess := auth.Session{Subject: "jm@example.com", Groups: []string{"hsa-users"}}
+	sess := auth.Session{Subject: "jm@example.com", Groups: []string{"hsa-users"}, IssuedAt: time.Now()}
 	cookie, err := auth.EncodeSession(sess, s.cfg.SessionKey)
 	if err != nil {
 		t.Fatal(err)