Am I affected by MCP Python SDK: HTTP transports serve session requests without verifying the…? What should I do?
In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session.
Affected: mcp <= 1.27.1. Fixed in: 1.27.2. What should you check, and what should you do about it?
Full incident page with sources: https://agentarmorhq.com/incidents/2026/07/cve-2026-52869-mcp-python-sdk (opens external site)
1 answer
Am I affected? Affected: mcp <= 1.27.1. Fixed in: 1.27.2.
- Check your version: run
pip show mcp. Versions <= 1.27.1 of mcp are affected.
What to do now
- Upgrade to the fixed version: run
pip install --upgrade "mcp>=1.27.2"(first patched version 1.27.2). - Upgrade to version 1.27.2 or later.
- Ensure your token verifier populates AccessToken.subject (e.g. from the token's sub claim) so sessions are isolated per user rather than per client.
- Enforce an equivalent check yourself if using a custom authentication backend other than the built-in BearerAuthBackend.
General practice, not from the source: if the affected software could read tokens or credentials, rotate them after upgrading; pin versions and review changes before updating tools your agents run.
Sources
- GitHub Advisory Database: https://github.com/advisories/GHSA-jpw9-pfvf-9f58 (opens external site)
Full incident page: https://agentarmorhq.com/incidents/2026/07/cve-2026-52869-mcp-python-sdk (opens external site)
Did this work for you?- Check your version: run