Model Context Protocol Python SDK: HTTP client transports followed cross-origin redirects carrying custom headers and bodies

Exploitation

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

CVSS

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

EPSS

• CVE-2026-116685: 0.40% estimated exploitation activity in the next 30 days; percentile 34.21 (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 SDK's Streamable HTTP and SSE client transports followed HTTP redirects to any host. If the configured MCP endpoint answered with a redirect to a different origin: - custom headers set on the client (for example an `X-API-Key` credential) were sent to that origin, and - on a `307` or `308` redirect the request body was re-sent there too. The SDK's OAuth client providers followed redirects on their own discovery, registration and token requests in the same way. A lone `Authorization` bearer header was not exposed; the HTTP library already drops it on cross-origin redirects. Anyone able to make an endpoint you trust redirect elsewhere (a stale redirect to a lapsed domain, a misrouted proxy rule, redirect logic an attacker can influence) could capture that material. A server that is itself malicious gains nothing this way; it already receives your requests directly.

Am I affected? Yes, if your code uses the SDK as an MCP client over HTTP through…

Published by the source

27 Oct 2026, 21:05 UTC

Last modified by the source

27 Oct 2026, 21:15 UTC

Source last checked

28 Oct 2026, 06:05 UTC

1. Revision 2 · amended · 28 Oct 2026

   Amended from osv record GHSA-5h93-6whr-6q8j

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-5h93-6whr-6q8j

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# Patch Next advisory brief: CVE-2026-116685

Advisory: Model Context Protocol Python SDK: HTTP client transports followed cross-origin redirects carrying custom headers and bodies
IDs: CVE-2026-116685, GHSA-5h93-6whr-6q8j
Record: https://patchnext.com/advisories/cve-2026-116685 (revision 2, source checked 28 Oct 2026, 06:05 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 2.2.0, 1.30.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mcp: affected >= 2.0.0, < 2.2.0; >= 1.8.0, < 1.30.0; fixed in 2.2.0, 1.30.0

## 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-5h93-6whr-6q8j (OSV record GHSA-5h93-6whr-6q8j lists affected ranges: PyPI mcp >=2.0.0 =1.8.0 <1.30.0 (fixed))
- Verified: https://github.com/advisories/GHSA-5h93-6whr-6q8j (GitHub-reviewed advisory GHSA-5h93-6whr-6q8j lists affected ranges: PyPI mcp >=2.0.0 =1.8.0 <1.30.0)

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