CVE-2026-52869

What it is

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.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. High risk of data exposure, data tampering.

Exploit odds

0.5% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-07-15 20:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-10-24 21:10 UTC Last updatedMost recent change to the record

Details

Published

2026-07-15 20:17 UTC

Last modified

2026-10-24 21:10 UTC

Exploit probability

0.5% · EPSS, next 30 days

Weakness

CWE-639 · Insecure direct object reference

Record status

Analyzed

Affected

• Lfprojects Mcp Python Sdk < 1.27.2

Attack profile CVSS 3.1 · 7.1

Attack vector

Network

Complexity

High

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

Low