CVE-2026-52869 Details
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.
A vulnerability exists in the MCP Python SDK (Model Context Protocol) in versions prior to 1.27.2. The issue arises in the SSE and stateful Streamable HTTP transports, where requests are routed to existing sessions based solely on the session ID. This routing occurs without verifying the authenticated principal that created the session. As a result, a client with a known session ID can inject JSON-RPC messages into that session, regardless of the bearer token used. This vulnerability allows for unauthorized message injection in sessions.
Upgrade to MCP Python SDK version 1.27.2 or later, which binds session IDs to the authenticated principal and rejects requests from different principals.
Metrics
CVSS 4.0 Severity and Vector Strings:
No CVSS 4.0 data is available for this CVE.
CVSS 3.x Severity and Vector Strings:
No data available for CVSS Version 2.0 on this CVE.
CISA-ADP
Assessed Jul 16, 2026References to Advisories, Solutions, and Tools
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Weakness Enumeration
| CWE-ID | CWE Name | Source |
|---|---|---|
| CWE-639 | Authorization Bypass Through User-Controlled Key | [email protected] |
Affected Products
| Product | Versions |
|---|---|
| lfprojects mcp python sdk | < 1.27.2 |
CPE
Remediation
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Change History
3 change records found show changes
| Date | Action | Recorded By |
|---|---|---|
| Jul 17, 2026 | Initial Analysis | [email protected] |
| Jul 16, 2026 | CVE Modified | CISA-ADP |
| Jul 15, 2026 | New CVE Received | [email protected] |