# CVE-2026-111771: MCP TypeScript SDK bearer authentication token validation bypass

Source: https://junglewise.ai/threats/cve-2026-111771-mcp-typescript-sdk-server-bearer-authentication-accepted-tokens  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-05. Updated: 2026-10-20  
License: CC BY 4.0 with a link back

CVE-2026-111771 · Severity: medium · CVSS 6.8 · Published 2026-11-05

The MCP TypeScript SDK's server-side bearer token authentication failed to validate that tokens were issued for the correct service. An attacker with a valid token for any service issued by the same authorization server could use it to authenticate to another service, gaining unauthorized access.

Technologies: [Anthropic @Modelcontextprotocol/Sdk](https://junglewise.ai/threats/technologies/anthropic-modelcontextprotocol-sdk), [@modelcontextprotocol/sdk (npm)](https://junglewise.ai/threats/technologies/modelcontextprotocol-sdk), [@modelcontextprotocol/server (npm)](https://junglewise.ai/threats/technologies/modelcontextprotocol-server). Vendors: [Anthropic](https://junglewise.ai/threats/vendors/anthropic), [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-111771 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-111771 being exploited in the wild.

**How severe is CVE-2026-111771?** Medium severity, with a CVSS score of 6.8, published 5 November 2026.

**Which products and versions are affected?** Anthropic @Modelcontextprotocol/Express 2.0.0 through 2.0.1; Anthropic @Modelcontextprotocol/Sdk 1.6.0 through 1.31.0; Anthropic @Modelcontextprotocol/Server 2.0.0 through 2.2.0; Anthropic @Modelcontextprotocol/Server-Legacy 2.0.0 through 2.3.0; and 4 more.

**Is there a fix for CVE-2026-111771?** Yes. @modelcontextprotocol/express (npm): upgrade to 2.0.2 or later; @modelcontextprotocol/sdk (npm): upgrade to 1.32.0 or later; @modelcontextprotocol/server (npm): upgrade to 2.3.0 or later; @modelcontextprotocol/server-legacy (npm): upgrade to 2.3.1 or later. Until then: If unable to upgrade immediately, reject any token whose audience (`aud`) claim does not include your server within the `verifyAccessToken()` function. When upgrading, enable the `expectedResource` setting by passing the audience your authorization server issues for your server and configure `verifyAccessToken()` to return the token's audience as `AuthInfo.resource`. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-111771 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-111771?** Check whether you run Anthropic @Modelcontextprotocol/Express, Anthropic @Modelcontextprotocol/Sdk, Anthropic @Modelcontextprotocol/Server or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- @modelcontextprotocol/express (npm): upgrade to 2.0.2 or later

  ```sh
  npm install @modelcontextprotocol/express@2.0.2
  ```

- @modelcontextprotocol/sdk (npm): upgrade to 1.32.0 or later

  ```sh
  npm install @modelcontextprotocol/sdk@1.32.0
  ```

- @modelcontextprotocol/server (npm): upgrade to 2.3.0 or later

  ```sh
  npm install @modelcontextprotocol/server@2.3.0
  ```

- @modelcontextprotocol/server-legacy (npm): upgrade to 2.3.1 or later

  ```sh
  npm install @modelcontextprotocol/server-legacy@2.3.1
  ```


If unable to upgrade immediately, reject any token whose audience (`aud`) claim does not include your server within the `verifyAccessToken()` function. When upgrading, enable the `expectedResource` setting by passing the audience your authorization server issues for your server and configure `verifyAccessToken()` to return the token's audience as `AuthInfo.resource`.

## Technical details

The SDK's `requireBearerAuth()` and `verifyBearerToken()` functions did not validate the token's audience (`aud`) claim, allowing tokens intended for one service to be accepted by another when multiple services share an authorization server. An unauthenticated attacker can exploit this by obtaining a valid token for any service and presenting it to the target service. The fix requires both upgrading and configuring the `expectedResource` setting while ensuring `verifyAccessToken()` returns the token's audience.

## Affected products

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

## Identifiers

- CVE-2026-111771
- GHSA-rvq5-wwqv-78pq

## Timeline

- 2026-10-20: disclosed: Vulnerability disclosed via GHSA advisory

## Related threats

- [MCP TypeScript server SDK cross-client message delivery in shared HTTP transport](https://junglewise.ai/threats/cve-2026-111770-mcp-typescript-server-sdk-shared-transport-or-server-instances)
- [MCP TypeScript client SDK credential leak via cross-origin redirects](https://junglewise.ai/threats/cve-2026-111769-mcp-typescript-client-sdk-http-client-transports-and-oauth)
- [MCP TypeScript SDK memory exhaustion via unbounded validation](https://junglewise.ai/threats/cve-2026-111768-mcp-typescript-sdk-one-http-request-within-the-default-size)
- [MCP TypeScript SDK denial of service via unbounded request body and JSON-RPC batch](https://junglewise.ai/threats/mcp-typescript-sdk-http-server-transports-and-handlers-did-not-limit-c51dda65)
- [MCP TypeScript SDK missing authorization in experimental tasks](https://junglewise.ai/threats/cve-2026-111767-mcp-typescript-sdk-experimental-tasks-were-not-tied-to-the)