CVE-2026-53710: mcp-contextforge-gateway has RestrictedPython sandbox bypass via getattr builtin in python_sandbox_server

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 1.1%, percentile 66: FIRST’s model estimates a 1.1% chance of exploitation activity in the next 30 days, higher than 66% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes mcp-contextforge-gateway (PyPI) <= 1.0.1, fixed in 1.0.2. Check your lockfile, including transitive dependencies.

What it is

MCP Context Forge is an AI gateway, registry, and proxy for MCP, A2A, REST, and gRPC APIs. Prior to 1.0.2, the pythonsandboxserver in mcp-servers/python/pythonsandboxserver/src/pythonsandboxserver/serverfastmcp.py exposes raw getattr through safebuiltins, omits a required getattr guard, and relies on validatecode checks for literal dangerous dunder strings. An attacker can construct dunder names at runtime, traverse the Python class hierarchy, reach subprocess.Popen, and execute OS commands with the server process privileges through the executecode MCP tool. The HTTP/SSE transport can expose this tool without authentication, while stdio-only deployments have reduced network reachability. The issue affects the pythonsandboxserver subproject and does not directly affect the core Context Forge gateway or proxy components. This issue is fixed in version 1.0.2.

From the GitHub advisory

Commit: f855e54d5b7bc1c91b977574a03b91eff6b86bb6 Component: mcp-servers/python/pythonsandboxserver/src/pythonsandboxserver/serverfastmcp.py

Vulnerability

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

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

Proof of Concept

Weakness type: CWE-94, CWE-693.

Product: mcp-contextforge-gateway (PyPI) · Assigned by: GitHub_M · Published: 2026-09-15

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 1.14% probability of exploitation activity in the next 30 days; percentile 66.

Timeline

• 2026-08-24 — GitHub-reviewed advisory GHSA-xm98-3vcf-fph7 published (critical)
• 2026-09-15 — CVE record last updated
• 2026-09-15 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-xm98-3vcf-fph7 github.com
2. CVE.org record CVE-2026-53710 cve.org
3. NVD entry CVE-2026-53710 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com

Critical PyPI advisories · Today’s Security Digest