CVE-2026-52869

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

Published Jul 15, 2026 CVSS 3.1 · 7.1 High EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Github GHSA Github GHSA GHSA-jpw9-pfvf-9f58 MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Modelcontextprotocol
Modelcontextprotocol python-sdk
Vendors & Products Modelcontextprotocol
Modelcontextprotocol python-sdk
Type Values Removed Values Added
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.
Title MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal
Weaknesses CWE-639
References
Metrics cvssV3_1

{'score': 7.1, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Jul 16, 2026, 1:00 PM
Reserved
Jun 8, 2026, 9:44 PM
Status
Analyzed
Published
Jul 15, 2026, 8:17 PM
Modified
Oct 24, 2026, 9:10 PM