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.
Published: 2026-06-18
Score: 9.3 Critical
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

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Remediation

No vendor fix or workaround currently provided.

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Tracking

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Advisories
Source ID Title
Github GHSA Github GHSA GHSA-wcpr-6g7x-p44r googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)
History

Sun, 21 Jun 2026 09:45:00 +0000

Type Values Removed Values Added
First Time appeared Google
Google mcp Toolbox For Databases
Vendors & Products Google
Google mcp Toolbox For Databases

Thu, 18 Jun 2026 18:15:00 +0000

Type Values Removed Values Added
Title Authentication Bypass via Opaque Token Validation in MCP Toolbox for Databases

Thu, 18 Jun 2026 16:45:00 +0000

Type Values Removed Values Added
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.
Weaknesses CWE-287
References
Metrics cvssV4_0

{'score': 9.3, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N'}

ssvc

{'options': {'Automatable': 'yes', 'Exploitation': 'none', 'Technical Impact': 'total'}, 'version': '2.0.3'}


Subscriptions

Google Mcp Toolbox For Databases
cve-icon MITRE

Status: PUBLISHED

Assigner: Google

Published:

Updated: 2026-06-18T13:53:14.985Z

Reserved: 2026-06-09T00:49:55.908Z

Link: CVE-2026-11718

cve-icon Vulnrichment

Updated: 2026-06-18T13:01:53.144Z

cve-icon NVD

No data.

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-06-20T22:56:18Z

Weaknesses