CVE-2026-11718: Critical severity Google mcp-toolbox vulnerability
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
go/github.com/googleapis/mcp-toolboxto a version that resolves this vulnerability.Fixed in 1.4.0
Event History
Frequently Asked Questions
What is CVE-2026-11718?
CVE-2026-11718 is an authentication bypass vulnerability in the googleapis/mcp-toolbox related to the opaque token validation process.
What is the severity of CVE-2026-11718?
The severity of CVE-2026-11718 is rated at 65, indicating a moderate risk.
How do I fix CVE-2026-11718?
To fix CVE-2026-11718, upgrade to the latest version of googleapis/mcp-toolbox where the vulnerability has been addressed.
What components are affected by CVE-2026-11718?
CVE-2026-11718 specifically affects the opaque token validation functionality within googleapis/mcp-toolbox.
When was CVE-2026-11718 published?
CVE-2026-11718 was published on June 18, 2026.