fix(csrf): clarify rejection diagnostics

api/routes.py

@@ -1260,8 +1260,37 @@ def _csrf_exempt_path(path: str) -> bool:
     return path in {"/api/auth/login", "/api/csp-report"}
 
 
+_CSRF_FAILURE_ATTR = "_hermes_csrf_failure_reason"
+
+
+def _set_csrf_failure_reason(handler, reason: str) -> bool:
+    try:
+        setattr(handler, _CSRF_FAILURE_ATTR, reason)
+    except Exception:
+        pass
+    return False
+
+
+def _clear_csrf_failure_reason(handler) -> None:
+    try:
+        if hasattr(handler, _CSRF_FAILURE_ATTR):
+            delattr(handler, _CSRF_FAILURE_ATTR)
+    except Exception:
+        pass
+
+
+def _csrf_rejection_error(handler) -> str:
+    reason = getattr(handler, _CSRF_FAILURE_ATTR, "")
+    if reason == "origin_mismatch":
+        return "Cross-origin mismatch - check reverse proxy headers"
+    if reason == "token_mismatch":
+        return "Session expired - reload the page"
+    return "Cross-origin request rejected"
+
+
 def _check_csrf(handler) -> bool:
     """Reject cross-origin or tokenless authenticated browser unsafe requests."""
+    _clear_csrf_failure_reason(handler)
     origin = handler.headers.get("Origin", "")
     referer = handler.headers.get("Referer", "")
     host = handler.headers.get("Host", "")
@@ -1271,7 +1300,7 @@ def _check_csrf(handler) -> bool:
     # Extract host:port from origin/referer
     m = _re.match(r"^https?://([^/]+)", target)
     if not m:
-        return False
+        return _set_csrf_failure_reason(handler, "origin_mismatch")
     origin_host = m.group(1)
     origin_scheme = m.group(0).split('://')[0].lower()  # 'http' or 'https'
     origin_name, origin_port = _normalize_host_port(origin_host)
@@ -1299,7 +1328,7 @@ def _check_csrf(handler) -> bool:
                 origin_allowed = True
                 break
     if not origin_allowed:
-        return False
+        return _set_csrf_failure_reason(handler, "origin_mismatch")
 
     from api.auth import CSRF_HEADER_NAME, is_auth_enabled, parse_cookie, verify_csrf_token
 
@@ -1307,7 +1336,9 @@ def _check_csrf(handler) -> bool:
         return True
     cookie_val = parse_cookie(handler)
     submitted = handler.headers.get(CSRF_HEADER_NAME) or handler.headers.get("X-CSRF-Token")
-    return verify_csrf_token(cookie_val or "", submitted or "")
+    if verify_csrf_token(cookie_val or "", submitted or ""):
+        return True
+    return _set_csrf_failure_reason(handler, "token_mismatch")
 
 
 def _client_ip_for_rate_limit(handler) -> str:
@@ -4616,7 +4647,7 @@ def handle_post(handler, parsed) -> bool:
         diag.stage("csrf")
     if not _csrf_exempt_path(parsed.path) and not _check_csrf(handler):
         try:
-            return j(handler, {"error": "Cross-origin request rejected"}, status=403)
+            return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
         finally:
             if diag:
                 diag.finish()
@@ -6196,7 +6227,7 @@ def handle_post(handler, parsed) -> bool:
 def handle_patch(handler, parsed) -> bool:
     """Handle all PATCH routes. Returns True if handled, False for 404."""
     if not _check_csrf(handler):
-        return j(handler, {"error": "Cross-origin request rejected"}, status=403)
+        return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
     body = read_body(handler)
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_patch
@@ -6211,7 +6242,7 @@ def handle_patch(handler, parsed) -> bool:
 def handle_delete(handler, parsed) -> bool:
     """Handle all DELETE routes. Returns True if handled, False for 404."""
     if not _check_csrf(handler):
-        return j(handler, {"error": "Cross-origin request rejected"}, status=403)
+        return j(handler, {"error": _csrf_rejection_error(handler)}, status=403)
     body = read_body(handler)
     if parsed.path.startswith("/api/kanban/"):
         from api.kanban_bridge import handle_kanban_delete

tests/test_issue2572_csrf_diagnostics.py

@@ -0,0 +1,74 @@
+"""Regression tests for issue #2572 CSRF rejection diagnostics."""
+
+import hmac
+import io
+import json
+import time
+from types import SimpleNamespace
+
+import api.auth as auth
+import api.routes as routes
+
+
+class _FakeHandler:
+    def __init__(self, headers=None, body=b"{}"):
+        self.headers = headers or {}
+        self.client_address = ("127.0.0.1", 12345)
+        self.rfile = io.BytesIO(body)
+        self.wfile = io.BytesIO()
+        self.status = None
+        self.sent_headers = {}
+
+    def send_response(self, status):
+        self.status = status
+
+    def send_header(self, key, value):
+        self.sent_headers[key] = value
+
+    def end_headers(self):
+        pass
+
+
+def _signed_cookie(raw_token: str) -> str:
+    sig = hmac.new(auth._signing_key(), raw_token.encode(), "sha256").hexdigest()
+    auth._sessions[raw_token] = time.time() + 60
+    return f"{raw_token}.{sig}"
+
+
+def _json_body(handler: _FakeHandler) -> dict:
+    return json.loads(handler.wfile.getvalue().decode("utf-8"))
+
+
+def test_origin_mismatch_csrf_rejection_has_diagnostic_error(monkeypatch):
+    monkeypatch.setattr(auth, "is_auth_enabled", lambda: False)
+    handler = _FakeHandler(
+        {
+            "Origin": "https://evil.example",
+            "Host": "127.0.0.1:8787",
+        }
+    )
+
+    routes.handle_post(handler, SimpleNamespace(path="/api/providers/delete"))
+
+    assert handler.status == 403
+    assert _json_body(handler)["error"] == "Cross-origin mismatch - check reverse proxy headers"
+
+
+def test_token_mismatch_csrf_rejection_has_reload_error(monkeypatch):
+    cookie = _signed_cookie("z" * 64)
+    monkeypatch.setattr(auth, "is_auth_enabled", lambda: True)
+    try:
+        handler = _FakeHandler(
+            {
+                "Origin": "http://127.0.0.1:8787",
+                "Host": "127.0.0.1:8787",
+                "Cookie": f"{auth.COOKIE_NAME}={cookie}",
+            }
+        )
+
+        routes.handle_post(handler, SimpleNamespace(path="/api/providers/delete"))
+
+        assert handler.status == 403
+        assert _json_body(handler)["error"] == "Session expired - reload the page"
+    finally:
+        auth._sessions.pop("z" * 64, None)