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
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.
Why this VPI (explainable, experimental)
VPI breakdown
| Impact | 71.00 |
| Exploitation signal(No additional exploitation signal) | ×1.00 |
| VPI | 71.00 |
VPI formula vpi-v1
| Source | CVSS Version | Base Score | Severity | Vector String | Assessment Date |
|---|---|---|---|---|---|
| OSV3rd | 3.1 | 7.1 | HIGH | CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L | 07/17/2026 |
| NVDNIST | 3.1 | 7.1 |
| CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L |
| 07/16/2026 |