Auth Bypass validateOpaqueToken in googleapis/mcp-toolbox
CVE-2026-11718 Published on June 18, 2026

An authentication bypass vulnerability exists in the generic opaque token validation path (validateOpaqueToken) of googleapis/mcp-toolbox. When the toolbox validates an opaque token via an OAuth 2.0 introspection endpoint (RFC 7662), it decodes the response into an introspectResp struct. However, the subsequent claim-checking logic (validateClaims) evaluates the issuer condition as if a.issuer != "" && iss != "". If the external OAuth provider's introspection response omits the optional iss (issuer) field completely, the variable iss defaults to an empty string. This causes the conditional block to evaluate to false and be skipped silently. Consequently, the application accepts tokens issued by unauthorized or unintended third-party identity providers.

NVD

Weakness Type

What is an authentification Vulnerability?

When an actor claims to have a given identity, the software does not prove or insufficiently proves that the claim is correct.

CVE-2026-11718 has been classified to as an authentification vulnerability or weakness.


Affected Versions

MCP Toolbox for Databases (googleapis/mcp-toolbox):

Exploit Probability

EPSS
0.20%
Percentile
10.40%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.