CVE-2026-52869: CWE-639: Authorization Bypass Through User-Controlled Key in modelcontextprotocol python-sdk
A vulnerability in the Model Context Protocol (MCP) Python SDK prior to version 1.27.2 allows authorization bypass. The issue occurs because certain transports route requests to sessions based solely on session identifiers without verifying the authenticated user who created the session. This enables an attacker with a known session ID and bearer token to inject JSON-RPC messages into another user's session. The vulnerability is fixed in version 1.27.2.
AI Analysis
Technical Summary
The MCP Python SDK (mcp on PyPI) versions before 1.27.2 have an authorization bypass vulnerability (CWE-639) in the SSE and stateful Streamable HTTP transports (mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager). These transports route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying that the authenticated principal owns the session. Consequently, a client authenticated with a bearer token who knows a valid session ID can inject JSON-RPC messages into that session. This issue is resolved in version 1.27.2.
Potential Impact
An attacker with a bearer token and knowledge of a valid session ID can bypass authorization controls to inject JSON-RPC messages into sessions belonging to other users. This can lead to unauthorized actions within the affected sessions, resulting in high confidentiality and integrity impact, and a low availability impact.
Mitigation Recommendations
Upgrade the MCP Python SDK to version 1.27.2 or later, where this authorization bypass vulnerability is fixed. No other mitigation or workaround is specified. Patch status is confirmed fixed in 1.27.2.
CVE-2026-52869: CWE-639: Authorization Bypass Through User-Controlled Key in modelcontextprotocol python-sdk
Description
A vulnerability in the Model Context Protocol (MCP) Python SDK prior to version 1.27.2 allows authorization bypass. The issue occurs because certain transports route requests to sessions based solely on session identifiers without verifying the authenticated user who created the session. This enables an attacker with a known session ID and bearer token to inject JSON-RPC messages into another user's session. The vulnerability is fixed in version 1.27.2.
CVSS v3.1
Score 7.1high
Affected software
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The MCP Python SDK (mcp on PyPI) versions before 1.27.2 have an authorization bypass vulnerability (CWE-639) in the SSE and stateful Streamable HTTP transports (mcp.server.sse.SseServerTransport and mcp.server.streamable_http_manager.StreamableHTTPSessionManager). These transports route requests to existing sessions using only the session_id query parameter or Mcp-Session-Id header without verifying that the authenticated principal owns the session. Consequently, a client authenticated with a bearer token who knows a valid session ID can inject JSON-RPC messages into that session. This issue is resolved in version 1.27.2.
Potential Impact
An attacker with a bearer token and knowledge of a valid session ID can bypass authorization controls to inject JSON-RPC messages into sessions belonging to other users. This can lead to unauthorized actions within the affected sessions, resulting in high confidentiality and integrity impact, and a low availability impact.
Mitigation Recommendations
Upgrade the MCP Python SDK to version 1.27.2 or later, where this authorization bypass vulnerability is fixed. No other mitigation or workaround is specified. Patch status is confirmed fixed in 1.27.2.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-06-08T21:44:27.363Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a57eb0b68715ace4363c891
Added to database: 07/15/2026, 20:18:19 UTC
Last enriched: 07/15/2026, 20:32:36 UTC
Last updated: 07/15/2026, 22:16:32 UTC
Views: 6
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