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Response snapshot Oct 27, 2026, 23:56 UTC

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VulnerabilityCVE-2026-116684

Model Context Protocol Python SDK: Server bearer-token authentication accepts tokens issued for a different resource server

In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server.

At a glance

Exploited
Not known
Severity
6.8 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-w4fh-qvv9-3v23 (opens a new tab)

What happened

Summary

In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server.

Am I affected?

Yes, if your HTTP server uses the SDK's bearer authentication (auth=AuthSettings(...) with token_verifier=), its authorization server also issues tokens for other services, and your TokenVerifier does not check the token's audience (aud) itself.

Continue reading

Affected versions:

- mcp 1.10.0 through 1.29.1 - mcp 2.0.0 through 2.1.1

Fix

Upgrade to:

- 1.x: mcp 1.30.0 or later - 2.x: mcp 2.2.0 or later

Upgrading alone changes nothing. The patched versions add a validate_token_resource setting that is off unless you set it. To turn the check on, make both of these changes together:

1. Set validate_token_resource=True in AuthSettings, with resource_server_url set to the audience your authorization server puts in tokens for your server. 2. Return the token's audience from verify_token() as AccessToken.resource.

A token for another service then gets 401. With only the first change, every request gets 401.

If your TokenVerifier already checks the audience, set validate_token_resource=False to silence the deprecation warning. The setting will default to True in 3.0.

If you cannot upgrade yet, reject any token whose aud does not include your server inside verify_token(). That closes the issue on every version.

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

6.8

Version 3.1As of

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

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-w4fh-qvv9-3v23 (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-w4fh-qvv9-3v23, 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: Server bearer-token authentication accepts tokens issued for a different resource server (CVE-2026-116684). https://kevwire.com/record/CVE-2026-116684

MLA

"Model Context Protocol Python SDK: Server bearer-token authentication accepts tokens issued for a different resource server (CVE-2026-116684)." KEVWire, 27 Oct. 2026, kevwire.com/record/CVE-2026-116684.

Chicago

KEVWire. "Model Context Protocol Python SDK: Server bearer-token authentication accepts tokens issued for a different resource server (CVE-2026-116684)." Last modified October 27, 2026. https://kevwire.com/record/CVE-2026-116684.

Evidence summary (plain text)

Model Context Protocol Python SDK: Server bearer-token authentication accepts tokens issued for a different resource server (Oct 27, 2026).
In affected versions, the SDK's server-side bearer authentication had no way to check that a token was issued for your server (its audience). Where one authorization server issues tokens for several services, a valid token for another service was also accepted by your MCP server.

Yes, if your HTTP server uses the SDK's bearer authentication (auth=AuthSettings(...) with token_verifier=), its authorization server also issues tokens for other services, and your TokenVerifier does not check the token's audience (aud) itself.
CVSS 6.8 (version 3.1; evidence as of Oct 27, 2026; https://github.com/advisories/GHSA-w4fh-qvv9-3v23).
Sources:
GitHub-reviewed advisory GHSA-w4fh-qvv9-3v23: https://github.com/advisories/GHSA-w4fh-qvv9-3v23
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-116684.
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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