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CVE-2026-11718 Google CVE debrief

A critical authentication bypass vulnerability exists in googleapis/mcp-toolbox. The vulnerability occurs in the generic opaque token validation path, allowing unauthorized third-party identity providers to issue accepted tokens. This happens when an external OAuth provider's introspection response omits the optional issuer (iss) field, causing the application to skip claim-checking logic silently. The CVSS score for this vulnerability is 9.3, indicating a high severity level. Organizations using googleapis/mcp-toolbox should take immediate action to mitigate this vulnerability.

Vendor
Google
Product
MCP Toolbox for Databases (googleapis/mcp-toolbox)
CVSS
CRITICAL 9.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-18
Original CVE updated
2026-06-22
Advisory published
2026-06-18
Advisory updated
2026-06-22

Who should care

Developers and administrators using googleapis/mcp-toolbox for authentication and authorization purposes should be aware of this critical vulnerability. This vulnerability can allow attackers to bypass authentication mechanisms, potentially leading to unauthorized access to sensitive data and systems.

Technical summary

The vulnerability exists in the validateOpaqueToken function of googleapis/mcp-toolbox. When validating an opaque token via an OAuth 2.0 introspection endpoint (RFC 7662), the application decodes the response into an introspectResp struct. However, the subsequent claim-checking logic (validateClaims) evaluates the issuer condition incorrectly if the iss field is omitted from the introspection response. This causes the conditional block to evaluate to false and be skipped, allowing tokens from unauthorized providers to be accepted.

Defensive priority

High

Recommended defensive actions

  • Update googleapis/mcp-toolbox to the latest version that includes the fix for this vulnerability.
  • Implement additional authentication and authorization mechanisms to verify the issuer of tokens.
  • Review and update OAuth 2.0 introspection endpoint configurations to ensure proper handling of the iss field.
  • Monitor and analyze authentication logs to detect potential exploitation attempts.
  • Consider implementing additional security measures, such as token blacklisting or revocation.
  • Restrict access to sensitive data and systems to prevent unauthorized access.
  • Perform regular security audits and vulnerability assessments to identify potential vulnerabilities.

Evidence notes

The information provided is based on the CVE-2026-11718 record and the NVD detail page. The vulnerability was published on June 18, 2026, and has a CVSS score of 9.3. The fix for this vulnerability is available in the googleapis/mcp-toolbox repository.

Official resources

public