---
id: "CVE-2026-116684"
kind: "vulnerability"
aliases: ["CVE-2026-116684", "GHSA-w4fh-qvv9-3v23"]
title: "Model Context Protocol Python SDK: Server bearer-token authentication accepts tokens issued for a different resource server"
severity: "medium"
cvss: 6.8
epss: null
exploited: false
status: "open"
category: "ai-tooling"
products: [{"name": "mcp", "fixed": ["2.2.0"], "affected": ">= 2.0.0, < 2.2.0", "ecosystem": "pypi"}, {"name": "mcp", "fixed": ["1.30.0"], "affected": ">= 1.10.0, < 1.30.0", "ecosystem": "pypi"}]
ecosystems: ["pypi"]
tags: ["ai-tooling", "cwe-863", "mcp", "python", "self-hosted"]
not_affected_if: ["default-config", "no-bearer-auth"]
published_at: "2026-10-27T20:17:13+00:00"
first_seen_at: "2026-10-27T20:33:27+00:00"
updated_at: "2026-10-27T21:09:20+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-116684", "first_seen_at": "2026-10-27T20:33:27+00:00"}, {"source": "ghsa", "url": "https://github.com/advisories/GHSA-w4fh-qvv9-3v23", "first_seen_at": "2026-10-27T21:09:20+00:00"}]
---

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

The official Model Context Protocol Python SDK's bearer authentication accepts a valid token issued for a different service when the token verifier does not check the aud audience claim. A token holder can authenticate to the wrong MCP server and use the access that token grants there.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- pypi:mcp affected >= 2.0.0, < 2.2.0 fixed 2.2.0
- pypi:mcp affected >= 1.10.0, < 1.30.0 fixed 1.30.0

## How to check

- Check the installed SDK version with `pip show mcp` and look for versions from `1.10.0` up to before `1.30.0`, or `2.0.0` through `2.1.x`.
- Search the server code for `AuthSettings` and `token_verifier` and confirm the verifier validates the `aud` claim.

## Fix

- Upgrade the `mcp` Python package to `1.30.0` or `2.2.0`, matching your major version.
- Make your `token_verifier` validate that the token's `aud` claim matches this MCP server.

## Timeline

- 2026-10-27T20:33:27+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-116684
- 2026-10-27T21:09:20+00:00 ghsa https://github.com/advisories/GHSA-w4fh-qvv9-3v23

## From the sources

### 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.

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_