Advisoriesby IbisRestrictedPython sandbox bypass via getattr builtin in python_sandbox_server
Title: RestrictedPython sandbox bypass via getattr() builtin + missing _getattr_ guard in python_sandbox_server
Severity: Critical 9.0 (HTTP transport) / High 8.8 (stdio floor)
CVSS 3.1: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
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:
```text
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"...
Affected versions
| Package | Affected | Fixed in |
|---|---|---|
| mcp-contextforge-gateway PyPI | < 1.0.2 | 1.0.2 |
Details and references
**Title:** RestrictedPython sandbox bypass via `getattr()` builtin + missing `_getattr_` guard in `python_sandbox_server` **Severity:** Critical 9.0 (HTTP transport) / High 8.8 (stdio floor) **CVSS 3.1:** `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H` **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: ```text 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: ```text 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.