CVE-2026-52869

HIGH

MEDIUM Priority: 0.04 BB Relevance: 54/100

Published: 2026-07-15 Modified: 2026-10-24 First seen: 2026-10-29 Status: active

Attack Surface

Network No Interaction Conf. Impact Integ. Impact

CVSS Vector Breakdown

AV: Network

AC: High

PR: Low

UI: None

S: Unchanged

C: High

I: High

A: Low

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.