CVE-2026-52869
Published:July 15, 2026
Updated:July 30, 2026
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
mcp (CONDA):
Affected version(s) >=1.3.0 <1.27.2Fix Suggestion:
Update to version 1.27.2mcp (CONDA):
Affected version(s) >=1.3.0 <1.27.2Fix Suggestion:
Update to version 1.27.2https://github.com/modelcontextprotocol/python-sdk.git (GITHUB):
Affected version(s) >=v1.0.0 <v1.27.2Fix Suggestion:
Update to version v1.27.2mcp (PYTHON):
Affected version(s) >=0.1.0 <1.27.2Fix Suggestion:
Update to version 1.27.2mcp (PYTHON):
Affected version(s) >=0.1.0 <1.27.2Fix Suggestion:
Update to version 1.27.2Related Resources (11)
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Contact UsCVSS v4
Base Score:
7.6
Attack Vector
NETWORK
Attack Complexity
HIGH
Attack Requirements
NONE
Privileges Required
LOW
User Interaction
NONE
Vulnerable System Confidentiality
HIGH
Vulnerable System Integrity
HIGH
Vulnerable System Availability
LOW
Subsequent System Confidentiality
NONE
Subsequent System Integrity
NONE
Subsequent System Availability
NONE
CVSS v3
Base Score:
7.1
Attack Vector
NETWORK
Attack Complexity
HIGH
Privileges Required
LOW
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
HIGH
Integrity
HIGH
Availability
LOW
Weakness Type (CWE)
Authorization Bypass Through User-Controlled Key
EPSS
Base Score:
0.32