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Response snapshot Oct 27, 2026, 23:56 UTC

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VulnerabilityCVE-2026-116686

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

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.

At a glance

Exploited
Not known
Severity
6.5 Medium (CVSS 3.1)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: mcp: 2.2.0; mcp: 1.30.0. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-rwrf-2pqf-9j8j (opens a new tab)

What happened

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?

Continue reading

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 #.

Affected package ranges reported by GitHub: mcp: >= 2.0.0, < 2.2.0 mcp: >= 1.10.0, < 1.30.0

Scores and how they were measured

CVSS base severity

6.5

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-rwrf-2pqf-9j8j (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-rwrf-2pqf-9j8j, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas (CVE-2026-116686). https://kevwire.com/record/CVE-2026-116686

MLA

"Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas (CVE-2026-116686)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-116686.

Chicago

KEVWire. "Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas (CVE-2026-116686)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-116686.

Evidence summary (plain text)

Model Context Protocol Python SDK: Client fetched server-chosen $ref URLs while validating tool results against output schemas (Oct 27, 2026).
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…
CVSS 6.5 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-rwrf-2pqf-9j8j).
Sources:
GitHub-reviewed advisory GHSA-rwrf-2pqf-9j8j: https://github.com/advisories/GHSA-rwrf-2pqf-9j8j
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-116686.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 23:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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