CVE-2026-52869
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 3.1
- 7.1
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L - EPSS
- 0.5% chance of exploitation in the next 30 days; higher than 44% of CVEs (scores of 2026-10-25)
- CWE
- CWE-639
- Published
- Modified
- Record from
- the NVD, synced
Affected
- lfprojects:mcp_python_sdk
Packages (OSV)
| Advisory | Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|---|
| GHSA-jpw9-pfvf-9f58 | PyPI | mcp | <1.27.2 | 1.27.2 |
| PYSEC-2026-3482 | PyPI | mcp | <1.27.2 | 1.27.2 |
References
- https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00
- https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a
- https://github.com/modelcontextprotocol/python-sdk/pull/2690
- https://github.com/modelcontextprotocol/python-sdk/pull/2719
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2