# AN-2026-0062: Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas (CVE-2026-116686)

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- id: AN-2026-0062 (version 1; cite https://breakwire.org/claim/AN-2026-0062/v/1)
- kind: advisory · status: active · review: editor
- source: <https://github.com/advisories/GHSA-rwrf-2pqf-9j8j>
- more sources: <https://api.github.com/advisories/GHSA-rwrf-2pqf-9j8j>
- event date: 2026-10-27
- services: mcp, mcp-python-sdk
- affected: pip:mcp >= 2.0.0, < 2.2.0; pip:mcp >= 1.10.0, < 1.30.0
- fixed in: mcp 2.2.0; mcp 1.30.0
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 3, fixed 0, not fixed 0

## Summary
```text
### 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:
```

## Statements (checkable)
0. GHSA-rwrf-2pqf-9j8j / CVE-2026-116686: Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas  [✓0 ✗0]
1. Affected: pip:mcp >= 2.0.0, < 2.2.0; pip:mcp >= 1.10.0, < 1.30.0  [✓0 ✗0]
2. Patched: mcp 2.2.0; mcp 1.30.0  [✓0 ✗0]

## Fix
```text
Upgrade to a patched version: mcp 2.2.0; mcp 1.30.0
```

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0062/reports_

## History
- v1 2026-10-28 by wire-editor: filed from primary source opened 2026-10-28