high CVSS 7.1

CVE-2026-52869·Python vulnerability

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.

Severity
high
Software
Python
Fixed in
1.27.2
Published
2026-07-15

Affected versions

Until: 1.27.2

Fixed in: 1.27.2

How to fix this CVE

Update the MCP Python SDK to version 1.27.2 or later to patch a session validation flaw in SSE and HTTP transport layers. This vulnerability allows authenticated users with knowledge of a session ID to inject messages into other users' sessions. Ensure all systems running the mcp package are upgraded immediately and restart any services using the SDK.

sudo dnf update python3-pip && pip3 install --upgrade mcp>=1.27.2

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET requests to MCP endpoints containing identical Mcp-Session-Id header or session_id=\w+ query parameter paired with different Authorization: Bearer tokens or X-Forwarded-For headers from distinct source IPs within short time intervals

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement session binding rules that verify the bearer token or client certificate matches the authenticated principal associated with each session ID before allowing JSON-RPC message routing. Reject requests where session_id or Mcp-Session-Id does not correlate to the current authenticated context.

How to check if you are affected

  1. Check installed mcp package version: pip3 show mcp | grep Version
  2. Verify if SSE or StreamableHTTPSessionManager is used: grep -r 'SseServerTransport\|StreamableHTTPSessionManager' /path/to/application --include='*.py'
  3. Review HTTP access logs for multiple requests with identical Mcp-Session-Id headers or session_id query parameters from different source IPs or bearer tokens
  4. Confirm patch applied: pip3 show mcp | grep 'Version: 1.27.2' or higher

FAQ

What is CVE-2026-52869?

CVE-2026-52869 is a session hijacking vulnerability in the MCP Python SDK where SSE and HTTP transports validate session IDs without confirming the authenticated user who owns the session, enabling cross-principal message injection attacks.

Is CVE-2026-52869 being actively exploited?

No, this vulnerability is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploit code is available, though the risk remains real for systems using vulnerable versions.

What versions of Python are affected by CVE-2026-52869?

All versions of the mcp package prior to 1.27.2 are affected. The vulnerability exists in the SSE and stateful HTTP transport implementations regardless of the Python interpreter version used.

How do I check if my server is vulnerable to CVE-2026-52869?

Run 'pip3 show mcp' to display the installed version. If the version is earlier than 1.27.2, your installation is vulnerable.

Does Defensia detect CVE-2026-52869?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the mcp package is installed on a monitored server, CVE-2026-52869 will appear in your dashboard with remediation steps.

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References

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