{"id":"CVE-2026-52869","aliases":["GHSA-jpw9-pfvf-9f58","PYSEC-2026-3482"],"url":"https://o3.security/vulnerability/CVE-2026-52869","summary":"MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal","details":"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.","published":"2026-07-15T20:06:23.604Z","modified":"2026-08-12T03:51:40.432810578Z","cvss":{"score":7.1,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L"},"epss":{"score":0.00535,"percentile":0.42826,"asOf":"2026-08-29"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"mcp","fixedVersion":"1.27.2"}],"fix":{"url":"https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00","label":"modelcontextprotocol/python-sdk@1abcca2"},"references":[{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/52xxx/CVE-2026-52869.json"},{"type":"ADVISORY","url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-52869"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/pull/2690"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/pull/2719"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:40.432810578Z"}}