MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.1 · CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:L

EPSS

• CVE-2026-52869: 0.53% estimated exploitation activity in the next 30 days; percentile 45.19 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session. Anyone who learned or guessed a session ID could send JSON-RPC messages on that session, regardless of which bearer token the request carried. Am I affected? Only if a developer's application server uses an HTTP transport (SSE, or Streamable HTTP in stateful mode) and authenticates requests. Servers on stdio, stateless Streamable HTTP, or with no authentication configured are not affected. Details Both transports look up the target session by its identifier alone — the sessionid query parameter for SSE (mcp.server.sse.SseServerTransport) and the Mcp-Session-Id header for Streamable HTTP (mcp.server.streamablehttpmanager.StreamableHTTPSessionManager). Once the lookup succeeded, the request was handled on that session without…

Published by the source

16 Jul 2026, 19:58 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-jpw9-pfvf-9f58

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-jpw9-pfvf-9f58

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# Patch Next advisory brief: CVE-2026-52869

Advisory: MCP Python SDK: HTTP transports serve session requests without verifying the authenticated principal
IDs: CVE-2026-52869, GHSA-jpw9-pfvf-9f58
Record: https://patchnext.com/advisories/cve-2026-52869 (revision 2, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 1.27.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mcp: affected <= 1.27.1; fixed in 1.27.2

## Suggested inspection (read-only)
- `python -m pip show mcp`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-jpw9-pfvf-9f58 (OSV record GHSA-jpw9-pfvf-9f58 lists affected ranges: PyPI mcp >=0 <1.27.2 (fixed))
- Verified: https://github.com/advisories/GHSA-jpw9-pfvf-9f58 (GitHub-reviewed advisory GHSA-jpw9-pfvf-9f58 lists affected ranges: PyPI mcp >=0 <=1.27.1)

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