{"id":"CVE-2026-116686","aliases":["EUVD-2026-111366","GHSA-rwrf-2pqf-9j8j"],"status":"published","title":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","description":"The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.","severity":{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-12","score":0.00337,"percentile":0.27443,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-918"],"published":"2026-10-27T19:29:16Z","modified":"2026-11-11T02:48:06Z","fixes":[{"ecosystem":"PyPI","package":"mcp","fixed_in":["2.2.0","1.30.0"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.0.0"},{"fixed":"2.2.0"}]}]},{"source":"ghsa","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"1.10.0"},{"fixed":"1.30.0"}]}]},{"source":"osv","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.0.0"},{"fixed":"2.2.0"}]}],"versions":["2.0.0","2.0.1","2.1.0","2.1.1"]},{"source":"osv","ecosystem":"PyPI","package":"mcp","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"1.10.0"},{"fixed":"1.30.0"}]}],"versions":["1.10.0","1.10.1","1.11.0","1.12.0","1.12.1","1.12.2","1.12.3","1.12.4","1.13.0","1.13.1","1.14.0","1.14.1","1.15.0","1.16.0","1.17.0","1.18.0","1.19.0","1.20.0","1.21.0","1.21.1","1.21.2","1.22.0","1.23.0","1.23.1","1.23.2","1.23.3","1.24.0","1.25.0","1.26.0","1.27.0","1.27.1","1.27.2","1.28.0","1.28.1","1.29.0","1.29.1"]}],"products":[{"vendor":"modelcontextprotocol","product":"python-sdk"}],"references":[{"url":"https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j","tags":["x_refsource_CONFIRM","advisory","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/pull/3394","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/pull/3396","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0","tags":["x_refsource_MISC","web"]},{"url":"https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0","tags":["x_refsource_MISC","web"]},{"url":"https://github.com/advisories/GHSA-rwrf-2pqf-9j8j"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/116xxx/CVE-2026-116686.json","tags":["advisory"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-116686","tags":["advisory"]},{"url":"https://github.com/modelcontextprotocol/python-sdk","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-116686","title":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","description":"The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.","status":"published","published":"2026-10-27T19:29:16.001Z","modified":"2026-10-28T16:13:45.473Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium","assessor":"cna"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:20:03.827373Z"}],"euvd":[{"id":"EUVD-2026-111366","description":"The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.","status":"published","published":"2026-10-27T19:29:16Z","modified":"2026-10-28T16:13:45Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium"}],"updated_at":"2026-11-02T16:59:20.41252Z"}],"ghsa":[{"id":"GHSA-rwrf-2pqf-9j8j","title":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","description":"### Summary\nIn affected versions, when a connected MCP server declared an `outputSchema` for a tool, the SDK's client validated each `call_tool` result with `jsonschema`'s default resolver, which fetches any `$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 `#`.","status":"published","published":"2026-10-27T21:05:29Z","modified":"2026-10-27T21:05:30Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:41:38.969831Z"}],"nvd":[{"id":"CVE-2026-116686","description":"The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.","status":"published","published":"2026-10-27T20:17:13.74Z","modified":"2026-10-28T17:18:03.663Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium","assessor":"secondary"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T14:13:04.037468Z"}],"osv":[{"id":"CVE-2026-116686","title":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","description":"The Model Context Protocol Python SDK is the official Python SDK for Model Context Protocol servers and clients. From 1.10.0 until 1.30.0 and 2.2.0, ClientSession, ClientSessionGroup, and Client tool calls validate server-declared outputSchema documents with jsonschema's default resolver, causing an unresolved $ref value to fetch an HTTP, HTTPS, or file URL from the client's network and filesystem context. A server can trigger blind requests or local-file reads and can stall every session in the process because resolution runs synchronously on the event loop without a timeout. This issue is fixed in versions 1.30.0 and 2.2.0.","status":"published","published":"2026-10-27T19:29:16.001Z","modified":"2026-11-11T10:47:26.519296484Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-11T03:22:28.447808Z"},{"id":"GHSA-rwrf-2pqf-9j8j","title":"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas","description":"### Summary\nIn affected versions, when a connected MCP server declared an `outputSchema` for a tool, the SDK's client validated each `call_tool` result with `jsonschema`'s default resolver, which fetches any `$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 `#`.","status":"published","published":"2026-10-27T21:05:29Z","modified":"2026-10-27T21:15:03.830660129Z","cvss":[{"version":"3.1","score":6.5,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H","level":"medium"}],"cwes":["CWE-918"],"updated_at":"2026-11-02T15:46:56.639206Z"}]}}