{"id":"CVE-2026-116686","aliases":["GHSA-rwrf-2pqf-9j8j"],"url":"https://o3.security/vulnerability/CVE-2026-116686","summary":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","details":"### Summary\nIn affected versions, when a connected MCP server declared an `outputSchema` for a tool, the SDK's client validated each `call_tool` result with `jsonschema`'s default resolver, which fetches any `$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:\n\n- 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\n- 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.\n\n### Am I affected?\nYes, 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 no setting that turned the validation off.\n\nAffected versions: 1.10.0 through 1.29.1, and 2.0.0 through 2.1.1.\n\nNot affected: applications that only build servers. If nothing in your code uses `ClientSession`, `ClientSessionGroup` or the 2.x `Client` class, you are not affected.\n\n### Fix\nUpgrade to `mcp` 1.30.0 (1.x) or 2.2.0 (2.x) or later. The client now resolves references only within the schema document the server sent (plus the standard JSON Schema metaschemas), so result validation performs no network or file access. A tool whose output schema points outside its own document fails at call time with `RuntimeError` (`Invalid schema for tool ...`); in-document references such as `#/$defs/...` work as before.\n\nUntil you can upgrade, call tools only on servers you trust. As a partial stopgap, skip tools whose `outputSchema` contains an `$id`, or a `$ref` / `$dynamicRef` that does not start with `#`.","published":"2026-10-27T19:29:16.001Z","modified":"2026-10-28T10:45:20.111194577Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"PyPI","name":"mcp","fixedVersion":"2.2.0"},{"ecosystem":"PyPI","name":"mcp","fixedVersion":"1.30.0"}],"fix":{"url":"https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836","label":"modelcontextprotocol/python-sdk@5ebdfed"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/116xxx/CVE-2026-116686.json"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3394"},{"type":"FIX","url":"https://github.com/modelcontextprotocol/python-sdk/pull/3396"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0"},{"type":"WEB","url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0"},{"type":"ADVISORY","url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-116686"},{"type":"PACKAGE","url":"https://github.com/modelcontextprotocol/python-sdk"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T10:45:20.111194577Z"}}