---
id: "CVE-2026-111771"
kind: "vulnerability"
aliases: ["CVE-2026-111771", "GHSA-rvq5-wwqv-78pq"]
title: "MCP TypeScript SDK: Server bearer authentication accepted tokens issued for another service"
severity: "medium"
cvss: 6.800000190734863
epss: null
exploited: false
status: "open"
category: "ai-tooling"
products: [{"name": "@modelcontextprotocol/sdk", "fixed": ["1.32.0"], "affected": ">= 1.6.0, < 1.32.0", "ecosystem": "npm"}, {"name": "@modelcontextprotocol/server", "fixed": ["2.3.0"], "affected": ">= 2.0.0, < 2.3.0", "ecosystem": "npm"}, {"name": "@modelcontextprotocol/express", "fixed": ["2.0.2"], "affected": ">= 2.0.0, < 2.0.2", "ecosystem": "npm"}, {"name": "@modelcontextprotocol/server-legacy", "fixed": ["2.3.1"], "affected": ">= 2.0.0, < 2.3.1", "ecosystem": "npm"}]
ecosystems: ["npm"]
tags: ["ai-tooling", "cwe-863", "mcp", "node", "oauth", "self-hosted", "typescript"]
not_affected_if: ["default-config-not-using-bearer-auth", "single-service-authorization-server", "stdio-transport"]
published_at: "2026-10-20T19:43:36+00:00"
first_seen_at: "2026-10-23T19:11:32+00:00"
updated_at: "2026-10-23T20:56:14+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-rvq5-wwqv-78pq", "first_seen_at": "2026-10-23T19:11:32+00:00"}]
---

# MCP TypeScript SDK: Server bearer authentication accepted tokens issued for another service

The MCP TypeScript SDK's server bearer-auth check did not verify that a token was issued for this server's audience, so a valid token issued for another service on the same authorization server was accepted. An attacker holding such a token from another service can call your MCP server as that user.

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

## Affected

- npm:@modelcontextprotocol/sdk affected >= 1.6.0, < 1.32.0 fixed 1.32.0
- npm:@modelcontextprotocol/server affected >= 2.0.0, < 2.3.0 fixed 2.3.0
- npm:@modelcontextprotocol/express affected >= 2.0.0, < 2.0.2 fixed 2.0.2
- npm:@modelcontextprotocol/server-legacy affected >= 2.0.0, < 2.3.1 fixed 2.3.1

## How to check

Check whether your HTTP MCP server uses requireBearerAuth() or verifyBearerToken() and whether verifyAccessToken() checks the aud claim.
Check installed versions: npm ls @modelcontextprotocol/sdk @modelcontextprotocol/server @modelcontextprotocol/express @modelcontextprotocol/server-legacy

## Fix

Upgrade @modelcontextprotocol/sdk to 1.32.0+, @modelcontextprotocol/server to 2.3.0+, @modelcontextprotocol/express to 2.0.2+, or @modelcontextprotocol/server-legacy to 2.3.1+; then pass expectedResource to requireBearerAuth() and return the token audience as AuthInfo.resource (both required).
Until upgraded, reject tokens whose aud does not include your server inside verifyAccessToken().

## Timeline

- 2026-10-23T19:11:32+00:00 ghsa https://github.com/advisories/GHSA-rvq5-wwqv-78pq

## 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 `requireBearerAuth()` or `verifyBearerToken()`, its authorization server also issues tokens for other services, and your `verifyAccessToken()` does not check the token's audience (`aud`) itself.

Affected versions:

- `@modelcontextprotocol/sdk` 1.6.0 through 1.31.0
- `@modelcontextprotocol/server` 2.0.0 through 2.2.0
- `@modelcontextprotocol/express` 2.0.0 through 2.0.1
- `@modelcontextprotocol/server-legacy` 2.0.0 through 2.3.0

### Fix
Upgrade to:

- 1.x: `@modelcontextprotocol/sdk` 1.32.0 or later
- 2.x: `@modelcontextprotocol/server` 2.3.0 or later, together with `@modelcontextprotocol/express` 2.0.2 or later if you use it
- `@modelcontextprotocol/server-legacy` 2.3.1 or later

**Upgrading alone changes nothing.** The patched versions add an `expectedResource` setting that is off by default. To turn the check on, make both of these changes together:

1. Pass the audience your authorization server puts in tokens for your server: `requireBearerAuth({ verifier, expectedResource: new URL('https://mcp.example.com/mcp') })`.
2. Return the token's audience from `verifyAccessToken()` as `AuthInfo.resource`, also a `URL`.

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

If you cannot upgrade yet, reject any token whose `aud` does not include your server inside `verifyAccessToken()`. 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_