fix: close rejected HTTP writes before reuse

api/routes.py

@@ -5871,6 +5871,9 @@ def _csrf_rejection_error(handler) -> str:
 def _check_csrf(handler) -> bool:
     """Reject cross-origin or tokenless authenticated browser unsafe requests."""
     if not _check_same_origin_browser_request(handler):
+        # CSRF checks run before read_body().  Close rather than reusing an
+        # HTTP/1.1 connection whose unread body would corrupt the next request.
+        handler.close_connection = True
         return False
     if not _is_browser_unsafe_request(handler):
         return True  # non-browser clients (curl, MCP, agent) have no Origin/Referer
@@ -5883,6 +5886,7 @@ def _check_csrf(handler) -> bool:
     submitted = handler.headers.get(CSRF_HEADER_NAME) or handler.headers.get("X-CSRF-Token")
     if verify_csrf_token(cookie_val or "", submitted or ""):
         return True
+    handler.close_connection = True
     return _set_csrf_failure_reason(handler, "token_mismatch")
 
 

server.py

@@ -404,7 +404,12 @@ class Handler(BaseHTTPRequestHandler):
             _is_csp_report_post = (
                 parsed.path == "/api/csp-report" and self.command == "POST"
             )
-            if not _is_csp_report_post and not check_auth(self, parsed): return
+            if not _is_csp_report_post and not check_auth(self, parsed):
+                # Authentication can reject an unsafe request before its body is
+                # consumed.  Reusing that HTTP/1.1 connection would make the
+                # unread body prefix the next request line.
+                self.close_connection = True
+                return
             result = route_func(self, parsed)
             if result is False:
                 return j(self, {'error': 'not found'}, status=404)

tests/test_rejected_write_connection_close.py

@@ -0,0 +1,55 @@
+"""Regression tests for HTTP/1.1 write rejection framing.
+
+A rejected POST can happen before its request body is consumed (authentication
+or CSRF failure). Keeping that connection alive leaves the body bytes in
+``rfile``; BaseHTTPRequestHandler then parses them as the next request method.
+The connection must close after those early rejections.
+"""
+from types import SimpleNamespace
+from typing import Any, cast
+
+
+def test_auth_rejected_write_closes_connection(monkeypatch):
+    import server
+
+    handler = SimpleNamespace(
+        path="/api/session/new",
+        command="POST",
+        close_connection=False,
+    )
+    monkeypatch.setattr(server, "reset_trusted_auth_request_state", lambda _handler: None)
+    monkeypatch.setattr(server, "get_profile_cookie", lambda _handler: None)
+    monkeypatch.setattr(server, "clear_request_profile", lambda: None)
+    monkeypatch.setattr(server, "check_auth", lambda _handler, _parsed: False)
+
+    def route_must_not_run(_handler, _parsed):
+        raise AssertionError("route ran after auth rejection")
+
+    server.Handler._handle_write(cast(Any, handler), route_must_not_run)
+
+    assert handler.close_connection is True
+
+
+def test_origin_rejected_csrf_closes_connection(monkeypatch):
+    import api.routes as routes
+
+    handler = SimpleNamespace(close_connection=False)
+    monkeypatch.setattr(routes, "_check_same_origin_browser_request", lambda _handler: False)
+
+    assert routes._check_csrf(handler) is False
+    assert handler.close_connection is True
+
+
+def test_token_rejected_csrf_closes_connection(monkeypatch):
+    import api.auth as auth
+    import api.routes as routes
+
+    handler = SimpleNamespace(headers={}, close_connection=False)
+    monkeypatch.setattr(routes, "_check_same_origin_browser_request", lambda _handler: True)
+    monkeypatch.setattr(routes, "_is_browser_unsafe_request", lambda _handler: True)
+    monkeypatch.setattr(auth, "is_auth_enabled", lambda: True)
+    monkeypatch.setattr(auth, "parse_cookie", lambda _handler: "session-cookie")
+    monkeypatch.setattr(auth, "verify_csrf_token", lambda _cookie, _token: False)
+
+    assert routes._check_csrf(handler) is False
+    assert handler.close_connection is True