Review reviewHigh

CVE-2026-52869

modelcontextprotocol python-sdk, mcp python sdk

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 ve...

CVSS: 7.1
EPSS: 0.32%
CISA KEV: Not listed
Published: 2026.07.16

PRIORITY ASSESSMENT

Review review

The CVSS severity warrants an early asset and exposure review.

Known exploitationNot established by KEV

Exploit probability0.32%

Technical severityCVSS 7.1

Vulnerability overview

Affected product and versions

Product: modelcontextprotocol python-sdk, mcp python sdk
Affected versions: >= < 1.27.2, < 1.27.2
Fixed versions: 1.27.2

Recommended response sequence

Confirm exposure before applying a vendor-supported change.

Full remediation guide

1
Identify
Confirm that modelcontextprotocol python-sdk, mcp python sdk and an affected version are present.
2
Prioritize
Combine exploitation signals with asset exposure and business criticality.
3
Remediate
Follow the vendor advisory or supported update path and preserve rollback options.
4
Verify
Recheck the version, service health, access paths, and relevant logs.

Technical data

CVSS vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L
CWE: CWE-639