high7.1
CVE-2026-52869
The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP).
Impact
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 packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| pip | mcp | <= 1.27.1 | 1.27.2 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Lfprojects | Mcp Python Sdk | before 1.27.2 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-jpw9-pfvf-9f58
- https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58
- https://nvd.nist.gov/vuln/detail/CVE-2026-52869
- https://github.com/modelcontextprotocol/python-sdk/pull/2690
- https://github.com/modelcontextprotocol/python-sdk/pull/2719
- https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00
- https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a
- https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2