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.
Affected products
lfprojects: mcp python sdk
References6
- github.com/modelcontextprotocol/python-sdk/security/advisories/… Vendor Advisory
- github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b… Patch
- github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515d… Patch
- github.com/modelcontextprotocol/python-sdk/pull/2690 Issue TrackingPatch
- github.com/modelcontextprotocol/python-sdk/pull/2719 Issue TrackingPatch
- github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2 Release Notes
Metrics
- CVSS 3.1
-
7.1
HIGH
Score source: CNA
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L - EPSS — exploit probability
- 1% percentile: 46 · as of 15 November 2026
- CWE
- CWE-639
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.