Vulnerability-Lookup

CVE-2026-116686 (GCVE-0-2026-116686)

Vulnerability from cvelistv5 – Published: 2026-10-27 19:29 – Updated: 2026-10-28 16:13

Title

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

Summary

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

6.5 (Medium)


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

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-10-28 14:34 UTC

CWE

CWE-918 - Server-Side Request Forgery (SSRF)

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7926 records in the last 180 days details

References

7 references

URL	Tags
https://github.com/modelcontextprotocol/python-sd…	x_refsource_CONFIRM
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC
https://github.com/modelcontextprotocol/python-sd…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
modelcontextprotocol	python-sdk	Affected: >= 1.10.0, < 1.30.0 Affected: >= 2.0.0, < 2.2.0	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-116686

Vulnerability from fkie_nvd - Published: 2026-10-27 20:17 - Updated: 2026-10-27 20:17

Severity

6.5 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/pull/3394
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/pull/3396
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0
	security-advisories@github.com	https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j

Impacted products

	Vendor	Product	Version

JSON

To clipboard

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    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-918"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

GHSA-RWRF-2PQF-9J8J

Vulnerability from github – Published: 2026-10-27 21:05 – Updated: 2026-10-27 21:05

Summary

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

Details

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 no setting that turned the validation off.

Affected versions: 1.10.0 through 1.29.1, and 2.0.0 through 2.1.1.

Not affected: applications that only build servers. If nothing in your code uses ClientSession, ClientSessionGroup or the 2.x Client class, you are not affected.

Fix

Upgrade 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.

Until 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 #.

Severity

6.5 (Medium)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.2.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.10.0"
            },
            {
              "fixed": "1.30.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-116686"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-918"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-27T21:05:29Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\nIn affected versions, when a connected MCP server declared an `outputSchema` for a tool, the SDK\u0027s client validated each `call_tool` result with `jsonschema`\u0027s default resolver, which fetches any `$ref` it cannot resolve locally by opening the URL. A server could therefore put an `http://`, `https://` or `file://` reference in a tool\u0027s output schema and make the client:\n\n- request that URL, or read that local file, from the client\u0027s own network position on every call of the tool (the content was used only as a schema and never sent back to the server), and\n- stall: the fetch ran synchronously on the event loop with no timeout, so a target that accepted the connection and then went quiet froze every session in the process for as long as it held the connection.\n\n### Am I affected?\nYes, if your application uses the SDK as an MCP client (`ClientSession` over any transport, `ClientSessionGroup`, or the 2.x `Client`) and calls tools on servers you do not fully control. There was no setting that turned the validation off.\n\nAffected versions: 1.10.0 through 1.29.1, and 2.0.0 through 2.1.1.\n\nNot affected: applications that only build servers. If nothing in your code uses `ClientSession`, `ClientSessionGroup` or the 2.x `Client` class, you are not affected.\n\n### Fix\nUpgrade to `mcp` 1.30.0 (1.x) or 2.2.0 (2.x) or later. The client now resolves references only within the schema document the server sent (plus the standard JSON Schema metaschemas), so result validation performs no network or file access. A tool whose output schema points outside its own document fails at call time with `RuntimeError` (`Invalid schema for tool ...`); in-document references such as `#/$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 `#`.",
  "id": "GHSA-rwrf-2pqf-9j8j",
  "modified": "2026-10-27T21:05:29Z",
  "published": "2026-10-27T21:05:29Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/security/advisories/GHSA-rwrf-2pqf-9j8j"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/pull/3394"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/pull/3396"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/commit/5ebdfed4060d8e5b11b8e2348f86bad4d1fb8836"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/commit/6705402e246dc4eb1fcdf4d9902b78d3c9c36e1b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/python-sdk"
    },
    {
      "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas"
}

Sightings

Author	Source	Type	Date	Other

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Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
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Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

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CVE-2026-116686 (GCVE-0-2026-116686)

FKIE_CVE-2026-116686

GHSA-RWRF-2PQF-9J8J

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Fix

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