CVE-2026-11718
Google Cloud Token Validation Authentication Bypass
Description
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.
INFO
Published Date :
June 18, 2026, 11:52 a.m.
Last Modified :
June 18, 2026, 11:52 a.m.
Remotely Exploit :
Yes !
Source :
Affected Products
The following products are affected by CVE-2026-11718
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 4.0 | CRITICAL | 14ed7db2-1595-443d-9d34-6215bf890778 |
Solution
- Update the googleapis/mcp-toolbox to the latest version.
- Ensure the toolbox handles missing 'iss' claims correctly.
- Implement strict issuer validation for all validated tokens.
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CVE-2026-11718 vulnerability anywhere in the article.