CVE-2026-11718

published 2026-06-18

CVE-2026-11718: An authentication bypass vulnerability exists in the generic opaque token validation path (validateOpaqueToken) of googleapis/mcp-toolbox. When the toolbox…

Priority P3 58 critical 9.3 CVSS 4.0

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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

2 ranges

Vendor	Product	Version range	Fixed in
github.com	googleapis_mcp-toolbox	>= 0 < 1.4.0	1.4.0
google	mcp_toolbox_for_databases	1.0.0 – 1.3.0	—

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CWE-287