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CVE-2026-53710 – Unauthenticated RCE via RestrictedPython Sandbox Bypass – IBM ContextForge MCP Gate

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Summary

CVE-2026-53710 is a critical remote code execution vulnerability in the python_sandbox_server component of IBM’s ContextForge MCP Gateway (mcp-contextforge-gateway). Three compounding weaknesses — an unmediated getattr in the sandbox’s safe builtins, weak literal-string validation, and an unauthenticated HTTP/SSE endpoint — allow a remote, unauthenticated attacker to escape the RestrictedPython sandbox and execute arbitrary OS commands with the privileges of the server process. The flaw carries a maximum CVSS v3.1 base score of 10.0.

Technical details

  • Root cause: The sandbox’s safe_builtins expose the raw getattr/setattr functions, bypassing RestrictedPython’s intended _getattr_ attribute-access mediation.
  • Validation weakness: Code submitted for execution is checked against a list of literal dangerous dunder strings (e.g., __class__, __subclasses__), but an attacker can construct these attribute names dynamically at runtime (e.g., via string concatenation), evading the filter entirely.
  • Exploitation path: Using unmediated getattr plus dynamically built dunder names, an attacker can traverse the Python class hierarchy from any object, reach subprocess.Popen (or equivalent), and execute arbitrary OS commands.
  • Attack vector: The execute_code MCP tool exposed by python_sandbox_server can be reachable over HTTP/SSE transport with no authentication layer, so any network client that can reach the endpoint can submit a malicious payload via a tools/call request.
  • Impact: Full compromise of confidentiality, integrity, and availability — arbitrary command execution at server-process privilege, which can lead to filesystem compromise or lateral network movement in containerized/networked deployments.

Affected software

  • IBM mcp-contextforge-gateway (ContextForge MCP Gateway), all releases up to and including 1.0.1, including the 0.x series and 1.0.0 pre-release/beta/RC builds, when the python_sandbox_server component’s execute_code tool is exposed over an unauthenticated HTTP/SSE transport.

Severity

  • CVSS v3.1 Base Score: 10.0 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade mcp-contextforge-gateway to version 1.0.2 or later, which removes the unmediated getattr/setattr exposure, replaces literal-string filtering with RestrictedPython’s stricter policy pipeline, and adds authentication to tools/call HTTP requests.
  • If patching is not immediately possible:
    • Do not expose the python_sandbox_server / execute_code tool over HTTP or SSE transport to untrusted networks.
    • Restrict deployment of the sandbox server to stdio-only transport where feasible.
    • Place any HTTP-exposed sandbox endpoints behind an authentication layer and network access controls (firewall/allowlist) until upgraded.
    • Note that IBM has since removed the sample Python Sandbox Server MCP server from the project entirely as it will not continue to be maintained; organizations relying on it should migrate away from this component.

References

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