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.
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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References
| Link | Providers |
|---|---|
| https://github.com/googleapis/mcp-toolbox/pull/3360 |
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History
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 |
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| Metrics |
cvssV4_0
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Status: PUBLISHED
Assigner: Google
Published:
Updated: 2026-06-18T13:53:14.985Z
Reserved: 2026-06-09T00:49:55.908Z
Link: CVE-2026-11718
Updated: 2026-06-18T13:01:53.144Z
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OpenCVE Enrichment
Updated: 2026-06-18T18:00:11Z
Weaknesses