Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas

Exploitation

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

CVSS

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

EPSS

• CVE-2026-116686: 0.34% estimated exploitation activity in the next 30 days; percentile 27.15 (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, when a connected MCP server declared an `outputSchema` for a tool, the SDK's client validated each `call_tool` result with `jsonschema`'s default resolver, which fetches any `$ref` it cannot resolve locally by opening the URL. A server could therefore put an `http://`, `https://` or `file://` reference in a tool's output schema and make the client: - request that URL, or read that local file, from the client's own network position on every call of the tool (the content was used only as a schema and never sent back to the server), and - stall: the fetch ran synchronously on the event loop with no timeout, so a target that accepted the connection and then went quiet froze every session in the process for as long as it held the connection.

Am I affected? Yes, if your application uses the SDK as an MCP client (`ClientSession` over any transport, `ClientSessionGroup`, or the 2.x `Client`) and calls tools on servers you do not fully control. There was…

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-rwrf-2pqf-9j8j

2. Revision 1 · created · 27 Oct 2026

   Published from ghsa record GHSA-rwrf-2pqf-9j8j

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# Patch Next advisory brief: CVE-2026-116686

Advisory: Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas
IDs: CVE-2026-116686, GHSA-rwrf-2pqf-9j8j
Record: https://patchnext.com/advisories/cve-2026-116686 (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 6.5)
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; >= 1.10.0, < 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-rwrf-2pqf-9j8j (OSV record GHSA-rwrf-2pqf-9j8j lists affected ranges: PyPI mcp >=2.0.0 =1.10.0 <1.30.0 (fixed))
- Verified: https://github.com/advisories/GHSA-rwrf-2pqf-9j8j (GitHub-reviewed advisory GHSA-rwrf-2pqf-9j8j lists affected ranges: PyPI mcp >=2.0.0 =1.10.0 <1.30.0)

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