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.

highCVSS 7.1CVE-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.

Details

Source: NVD Recent CVEs (US · database · site)
Severity: high — CVSS 7.1
Published: 2026-07-15
Last updated: 2026-07-17
Exploitation: Not in CISA KEV at last sync

Original advisory: https://nvd.nist.gov/vuln/detail/CVE-2026-52869

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-528690.32% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 24% of all scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-52869	coverage & exploitation status	NVD · CVE.org

Same CVEs, other sources

How other CERTs, PSIRTs and databases cover the vulnerabilities in this advisory.

highGHSA-jpw9-pfvf-9f58: MCP Python SDK: HTTP transports serve session requests without verifying the authenticate…ghsa

Recent advisories for MCP Python SDK

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

highGHSA-vj7q-gjh5-988w: MCP Python SDK: WebSocket server transport does not support Host/Origin validationghsa · 2026-07-16
highGHSA-jpw9-pfvf-9f58: MCP Python SDK: HTTP transports serve session requests without verifying the authenticate…ghsa · 2026-07-16
highGHSA-hvrp-rf83-w775: MCP Python SDK: Experimental task handlers allow any client to access and cancel other cl…ghsa · 2026-07-16
highCVE-2026-59950: The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protoc…nvd · 2026-07-15
highCVE-2026-52870: The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protoc…nvd · 2026-07-15