What’s Vulnerable › Vulnerabilities

CVE-2026-52869: MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

• Severity: high (CVSS 7.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-07-15 · updated 2026-10-24
• Weakness: CWE-639

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

• modelcontextprotocol python-sdk: before 1.27.2 — fixed in 1.27.2

How to fix

Upgrade to a fixed version: modelcontextprotocol python-sdk 1.27.2. Check the vendor advisory for mitigations.

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
• https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58

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