Auth bypass in Lfprojects Mcp Python Sdk
CVE-2026-52869
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.
Vulnerability class: IDOR (Insecure Direct Object Reference)
EPSS: 0.005 (42.8th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 7.1 (High). Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L.
Affected products
- Lfprojects Mcp_python_sdk
- Modelcontextprotocol Python-sdk — versions < 1.27.2
Weakness classification (CWE)
References
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC, Issue Tracking)
- security-advisories@github.com (Patch, x_refsource_MISC, Issue Tracking)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-52869?
- CVE-2026-52869 is a high-severity vulnerability in Lfprojects Mcp Python Sdk, classified under Authorization Bypass Through User-Controlled Key. CVSS score: 7.1/10. Published 2026-07-15.
- How severe is CVE-2026-52869?
- High severity. CVSS v3 base score is 7.1 out of 10.