EUVD-2026-37880Published

An authentication bypass vulnerability exists in the generic opaque token validation path (validateOpaqueToken) of googleapis/mcp-toolbox.

Exact source revision recorded 28 Oct 2026 00:57:05 UTC. It does not establish the source's current position. Latest evidence.

Recorded JSON

euvd · EUVD-2026-37880

Published

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.

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datePublished: Jun 18, 2026, 11:52:42 AM

dateUpdated: Jun 18, 2026, 1:53:14 PM

Source published: Jun 18, 2026, 11:52:42 AM (timezone not stated)
Source modified: Jun 18, 2026, 1:53:14 PM (timezone not stated)
Observed here: 28 Oct 2026 00:57:05 UTC
Source terms: ENISA EUVD · source terms and attribution apply

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EUVD CVSS 4.0 · 9.3 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