CVE-2026-52869

High

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.

SeverityHigh

CVSS7.1

Exploit probability—

ProductLfprojects Mcp Python Sdk

PublishedJul 15, 2026

First seenJul 15, 2026

Weakness typeCWE-639Authorization bypass through user-controlled key

How to fix

Update

lfprojects mcp_python_sdk1.27.2 or later
mcp1.27.2 or later

Affected products

Product	Affected versions	Fixed in
lfprojects mcp_python_sdk	v < 1.27.2	1.27.2
mcpPyPI	v < 1.27.2	1.27.2

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