CVE-2026-11718

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.

CVSS 4.0

9.3CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS 3.1

9.1CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

0.2% chance of exploitation in the next 30 days; higher than 8% of CVEs (scores of 2026-10-25)

CWE

CWE-287

Published

2026-06-18

Modified

2026-10-26

Record from

the NVD, synced 2026-10-26

Affected

• google:mcp_toolbox_for_databases

Packages (OSV)

References

• https://github.com/googleapis/mcp-toolbox/pull/3360

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