CVE-2026-52869

HIGH 7.1

MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

CVSS Score

7.1

EPSS Score

0.0%

EPSS Percentile

0th

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.

CWE	CWE-639
Vendor	modelcontextprotocol
Product	python-sdk
Published	Jul 15, 2026
Last Updated	Jul 16, 2026

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CVSS v3 Breakdown

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

Attack Vector

Network

Attack Complexity

High

Privileges Required

Low

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

Low

Affected Versions

modelcontextprotocol / python-sdk

< 1.27.2

References

NVD ↗ CVE.org ↗ EPSS Data ↗

github.com: https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-jpw9-pfvf-9f58 github.com: https://github.com/modelcontextprotocol/python-sdk/pull/2690 github.com: https://github.com/modelcontextprotocol/python-sdk/pull/2719 github.com: https://github.com/modelcontextprotocol/python-sdk/commit/1abcca2408a6b50e10ec601181f63f9978705c00 github.com: https://github.com/modelcontextprotocol/python-sdk/commit/ce267b6fc515dc4efc1dc70b6975b16ff0feef0a github.com: https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.27.2