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Authorization Bypass Through User-Controlled Key

CVE-2026-52869

High
7.1/10

Summary

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.

  • HIGH
  • NETWORK
  • HIGH
  • UNCHANGED
  • NONE
  • LOW
  • HIGH
  • LOW

CWE-639 - Authorization Bypass Through User-Controlled Key

The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.

References

Advisory Timeline

  • Published