PYSEC-2026-3862

Commit: f855e54d5b7bc1c91b977574a03b91eff6b86bb6 Component: mcp-servers/python/python_sandbox_server/src/python_sandbox_server/server_fastmcp.py

Vulnerability

RestrictedPython's sandbox in ContextForge's python_sandbox_server sub-project allows arbitrary Python execution via three compounding weaknesses:

1. Raw getattr is exposed in safe_builtins, bypassing _getattr_ mediation.
2. validate_code checks for literal dangerous dunder strings, but the payload constructs those names at runtime.
3. The execute_code MCP tool can be exposed over HTTP/SSE transport with no authentication layer.

Proof of Concept

The local PoC was executed against the real pinned sandbox code path. It constructs dunder names at runtime, walks Python's class hierarchy through the exposed getattr, finds subprocess.Popen, and executes a harmless marker command.

Transcript excerpt:

POC: IBM/mcp-context-forge RestrictedPython getattr sandbox bypass
validation={'valid': True, 'message': 'Code passed validation', 'warnings': None}
success=True
stdout="FOUND_POPEN\nb'IBM_SANDBOX_POC_PASS'\n"
IBM_RESTRICTEDPYTHON_GETATTR_POPEN_REPRO_PASS

PoC artifact hashes:

beb2d856c5a1b7c10005c9c0fccaf3d491d4fbcc2a69bc54b5e95c5730c0874b  run.py
1037dcb4f0831a3c96ca728527f939af1d3e19514b23ef54f663707875748ff8  run.sh
7026e676ee31916886f10feed959ffdbe486fda05b17421387e4717acbf90ba6  transcript.txt

Production Deployment Context

ContextForge's documentation describes registering MCP servers as gateways in a running ContextForge instance. If this sandbox server is reachable over HTTP in that pattern, any client that can reach the endpoint can submit the payload through tools/call.

Impact

An attacker with access to the python_sandbox_server HTTP endpoint can execute OS commands with the privileges of the server process. In containerized deployments with host mounts or internal network reachability, this can become filesystem compromise or network pivoting from the gateway host.

Suggested Fix

Remove raw getattr and setattr from safe_builtins, expose only policy-controlled attribute access if dynamic lookup is required, replace substring filtering with a stricter RestrictedPython policy pipeline, and add authentication to HTTP transport for all tools/call requests.

Caveats

This report covers the python_sandbox_server sub-project. The core ContextForge gateway and proxy components are not directly affected. The Critical score assumes HTTP reachability; stdio-only deployments reduce the attack vector.