MCP Python SDK external output-schema reference retrieval advisory

Evidence record

Python MCP SDK outputSchema external-reference advisory covering file/HTTP schema reads and unbounded synchronous resolution; patched in mcp 1.30.0/2.2.0. Schema contents were not automatically returned.

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Claim-level citations (3)

• supportsMCP output-schema validation could make the client fetch server-selected URLs and files: CVE-2026-116686 identifies http, https and file external references fetched by the Python MCP client during schema validation.

  Advisory summary; default JSON Schema resolver

• supportsMCP output-schema validation could make the client fetch server-selected URLs and files: The advisory states the read schema data was used for validation rather than automatically returned to the server; a hanging reference could still block the event loop without a timeout.

  Advisory; consequence and client scope

• supportsMCP output-schema validation could make the client fetch server-selected URLs and files: An external output-schema reference could synchronously stall the MCP client event loop without a timeout, but the fetched schema bytes were used for validation rather than automatically sent back to the server.

  Summary and Am I affected?, GHSA-rwrf-2pqf-9j8j