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
npm install @modelcontextprotocol/express@2.0.2 - @modelcontextprotocol/sdk (npm): upgrade to 1.32.0 or later
npm install @modelcontextprotocol/sdk@1.32.0 - @modelcontextprotocol/server (npm): upgrade to 2.3.0 or later
npm install @modelcontextprotocol/server@2.3.0 - @modelcontextprotocol/server-legacy (npm): upgrade to 2.3.1 or later
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