fixup: address Nora's review on #520 (security blockers)

- frontend/login: derive cookie `secure` flag from request URL protocol.
  Pre-PR the cookie was only read by SSR so the flag didn't matter; now
  the cookie IS the API credential and must be Secure on HTTPS or it
  leaks a 24h Basic token on plaintext networks. Dev runs over HTTP and
  would silently lose the cookie if we hardcoded `secure: true`, so the
  flag follows `event.url.protocol === 'https:'`.

- SecurityConfig: rewrite the CSRF-disabled comment. The old
  "browsers block cross-origin custom headers" justification no longer
  holds once /api/* is authenticated via the cookie. Make the
  load-bearing dependencies explicit: SameSite=strict on the auth_token
  cookie + Spring's default CORS rejection.

- AuthTokenCookieFilter:
  - Scope to /api/* only. /actuator/health and similar must not be
    cookie-authenticated.
  - Refuse malformed percent-encoding (URLDecoder throws); forward the
    request without a promoted Authorization rather than crash.
  - Use isBlank() instead of isEmpty() per Nora.
  - Javadoc warning: getHeaderNames/getHeaders exposes the Basic
    credential; any future header-iterating logger must scrub
    Authorization before logging.

- Tests: add `passes_through_unchanged_when_request_is_outside_api_scope`
  (/actuator/health with cookie should NOT be wrapped) and
  `passes_through_unchanged_when_cookie_value_is_malformed_percent_encoding`.
  Tighten the explicit-header test to verify same-instance forwarding
  rather than just header equality.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/security/AuthTokenCookieFilter.java

@@ -39,17 +39,36 @@ import java.util.Enumeration;
  *
  * <p>See #520. Filter runs at {@code Ordered.HIGHEST_PRECEDENCE} so it
  * mutates the request before any Spring Security filter sees it.
+ *
+ * <p><b>Scope:</b> only {@code /api/*} requests are touched. The
+ * {@code /actuator/*} block in Caddy plus the open auth/reset paths in
+ * {@link SecurityConfig} must NOT receive a promoted Authorization.
+ *
+ * <p><b>⚠ Log-leakage warning:</b> the wrapped request exposes the
+ * Authorization header via {@code getHeaderNames}/{@code getHeaders}. Any
+ * filter or interceptor that iterates request headers will see the live
+ * Basic credential. Do NOT add a request-header logger downstream of this
+ * filter without explicitly scrubbing the {@code Authorization} field.
  */
 @Component
 @Order(org.springframework.core.Ordered.HIGHEST_PRECEDENCE)
 public class AuthTokenCookieFilter extends OncePerRequestFilter {
 
     static final String COOKIE_NAME = "auth_token";
+    static final String SCOPE_PREFIX = "/api/";
 
     @Override
     protected void doFilterInternal(HttpServletRequest request,
                                     HttpServletResponse response,
                                     FilterChain chain) throws ServletException, IOException {
+        // Scope: only /api/* needs cookie promotion. /actuator/health (open),
+        // /api/auth/forgot-password (open), /login etc. don't.
+        if (!request.getRequestURI().startsWith(SCOPE_PREFIX)) {
+            chain.doFilter(request, response);
+            return;
+        }
+        // An explicit Authorization header wins — this is the SSR fetch path
+        // (hooks.server.ts builds the header itself).
         if (request.getHeader(HttpHeaders.AUTHORIZATION) != null) {
             chain.doFilter(request, response);
             return;
@@ -60,8 +79,17 @@ public class AuthTokenCookieFilter extends OncePerRequestFilter {
             return;
         }
         for (Cookie c : cookies) {
-            if (COOKIE_NAME.equals(c.getName()) && c.getValue() != null && !c.getValue().isEmpty()) {
-                String decoded = URLDecoder.decode(c.getValue(), StandardCharsets.UTF_8);
+            if (COOKIE_NAME.equals(c.getName()) && c.getValue() != null && !c.getValue().isBlank()) {
+                String decoded;
+                try {
+                    decoded = URLDecoder.decode(c.getValue(), StandardCharsets.UTF_8);
+                } catch (IllegalArgumentException malformed) {
+                    // Malformed percent-encoding — refuse to forward a bogus
+                    // Authorization header. Spring Security will treat the
+                    // request as unauthenticated.
+                    chain.doFilter(request, response);
+                    return;
+                }
                 chain.doFilter(new AuthHeaderRequest(request, decoded), response);
                 return;
             }

backend/src/main/java/org/raddatz/familienarchiv/security/SecurityConfig.java

@@ -37,12 +37,20 @@ public class SecurityConfig {
     @Bean
     public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
         http
-                // CSRF is intentionally disabled: every request from the SvelteKit frontend
-                // carries an explicit Authorization header (Basic Auth token injected by
-                // hooks.server.ts). Browsers block cross-origin requests from setting custom
-                // headers, so cross-site request forgery via a third-party page is not
-                // possible with this auth scheme. If the auth model ever changes to
-                // cookie-based sessions, CSRF protection must be re-enabled.
+                // CSRF is intentionally disabled. With the cookie-promotion model
+                // (auth_token cookie → Authorization header via AuthTokenCookieFilter,
+                // see #520), every authenticated request to /api/* now carries the
+                // credential automatically once the cookie is set. The CSRF defence
+                // for state-changing endpoints is therefore LOAD-BEARING on:
+                //
+                //   1. SameSite=strict on the auth_token cookie (login/+page.server.ts).
+                //      A cross-site POST from evil.com cannot include the cookie.
+                //   2. CORS — Spring's default rejects cross-origin requests with
+                //      credentials unless explicitly allowed (no allowedOrigins config).
+                //
+                // If either of those is ever weakened (e.g. cookie flipped to
+                // SameSite=lax, CORS allowedOrigins expanded), CSRF protection
+                // MUST be re-enabled here.
                 .csrf(csrf -> csrf.disable())
 
                 .authorizeHttpRequests(auth -> {

backend/src/test/java/org/raddatz/familienarchiv/security/AuthTokenCookieFilterTest.java

@@ -29,6 +29,7 @@ class AuthTokenCookieFilterTest {
     @Test
     void promotes_url_encoded_auth_token_cookie_to_decoded_Authorization_header() throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
         req.setCookies(new Cookie("auth_token", "Basic%20YWRtaW5AZmFtaWx5YXJjaGl2ZS5sb2NhbDpzZWNyZXQ%3D"));
         MockHttpServletResponse res = new MockHttpServletResponse();
         FilterChain chain = mock(FilterChain.class);
@@ -47,6 +48,7 @@ class AuthTokenCookieFilterTest {
     @Test
     void preserves_explicit_Authorization_header_and_ignores_cookie() throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
         req.addHeader("Authorization", "Basic explicit-header-wins");
         req.setCookies(new Cookie("auth_token", "Basic%20cookie-would-have-promoted"));
         MockHttpServletResponse res = new MockHttpServletResponse();
@@ -54,15 +56,14 @@ class AuthTokenCookieFilterTest {
 
         filter.doFilter(req, res, chain);
 
-        ArgumentCaptor<HttpServletRequest> captor = ArgumentCaptor.forClass(HttpServletRequest.class);
-        verify(chain).doFilter(captor.capture(), org.mockito.ArgumentMatchers.any(HttpServletResponse.class));
-        assertThat(captor.getValue().getHeader("Authorization"))
-            .isEqualTo("Basic explicit-header-wins");
+        // Forwards the original request unchanged — same instance, no wrapping.
+        verify(chain).doFilter(req, res);
     }
 
     @Test
     void passes_through_when_no_cookies_at_all() throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
         MockHttpServletResponse res = new MockHttpServletResponse();
         FilterChain chain = mock(FilterChain.class);
 
@@ -74,6 +75,7 @@ class AuthTokenCookieFilterTest {
     @Test
     void passes_through_when_auth_token_cookie_is_absent() throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
         req.setCookies(new Cookie("some_other_cookie", "value"));
         MockHttpServletResponse res = new MockHttpServletResponse();
         FilterChain chain = mock(FilterChain.class);
@@ -86,6 +88,7 @@ class AuthTokenCookieFilterTest {
     @Test
     void passes_through_when_auth_token_cookie_is_empty() throws Exception {
         MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
         req.setCookies(new Cookie("auth_token", ""));
         MockHttpServletResponse res = new MockHttpServletResponse();
         FilterChain chain = mock(FilterChain.class);
@@ -94,4 +97,38 @@ class AuthTokenCookieFilterTest {
 
         verify(chain).doFilter(req, res);
     }
+
+    @Test
+    void passes_through_unchanged_when_request_is_outside_api_scope() throws Exception {
+        MockHttpServletRequest req = new MockHttpServletRequest();
+        // /actuator/health and similar must NOT receive a promoted Authorization
+        // header — they have their own access rules and should never be authed
+        // via the cookie.
+        req.setRequestURI("/actuator/health");
+        req.setCookies(new Cookie("auth_token", "Basic%20YWR=="));
+        MockHttpServletResponse res = new MockHttpServletResponse();
+        FilterChain chain = mock(FilterChain.class);
+
+        filter.doFilter(req, res, chain);
+
+        // Forwards the original request unchanged — same instance, no wrapping.
+        verify(chain).doFilter(req, res);
+    }
+
+    @Test
+    void passes_through_unchanged_when_cookie_value_is_malformed_percent_encoding() throws Exception {
+        MockHttpServletRequest req = new MockHttpServletRequest();
+        req.setRequestURI("/api/users/me");
+        // Lone "%" without two hex digits → URLDecoder throws → filter must
+        // refuse to forward a bogus Authorization header.
+        req.setCookies(new Cookie("auth_token", "Basic%2"));
+        MockHttpServletResponse res = new MockHttpServletResponse();
+        FilterChain chain = mock(FilterChain.class);
+
+        filter.doFilter(req, res, chain);
+
+        // Forwards the original request unchanged — Spring Security treats it
+        // as unauthenticated rather than crashing on bad input.
+        verify(chain).doFilter(req, res);
+    }
 }