Vulnerability Description
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.27.2, the SSE and stateful Streamable HTTP transports mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying the authenticated principal that created the session, allowing a different bearer-token-authenticated client with a known session ID to inject JSON-RPC messages into that session. This issue is fixed in version 1.27.2.
CVSS Score
HIGH
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Lfprojects | Mcp Python Sdk | < 1.27.2 |
Related Weaknesses (CWE)
References
- https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec60Patch
- https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc7Patch
- https://github.com/modelcontextprotocol/python-sdk/pull/2690Issue TrackingPatch
- https://github.com/modelcontextprotocol/python-sdk/pull/2719Issue TrackingPatch
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2Release Notes
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9Vendor Advisory
FAQ
What is CVE-2026-52869?
CVE-2026-52869 is a vulnerability with a CVSS score of 7.1 (HIGH). The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to 1.27.2, the SSE and stateful Streamable HTTP transports mcp.server.sse.SseServerTranspo...
How severe is CVE-2026-52869?
CVE-2026-52869 has been rated HIGH with a CVSS base score of 7.1/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-52869?
Check the references section above for vendor advisories and patch information. Affected products include: Lfprojects Mcp Python Sdk.