Google MCP Toolbox for Databases up to 1.3.0 iss improper authentication

Summaryinfo

A vulnerability was found in Google MCP Toolbox for Databases up to 1.3.0. It has been declared as critical. Affected by this vulnerability is an unknown functionality. Executing a manipulation of the argument iss can lead to improper authentication. This vulnerability is handled as CVE-2026-11718. The attack can be executed remotely. There is not any exploit available.

Detailsinfo

A vulnerability classified as critical was found in Google MCP Toolbox for Databases up to 1.3.0. This vulnerability affects some unknown processing. The manipulation of the argument iss with an unknown input leads to a improper authentication vulnerability. The CWE definition for the vulnerability is CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

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.

The weakness was released by Haonh. The advisory is shared for download at github.com. This vulnerability was named CVE-2026-11718 since 06/09/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $5k-$25k (estimation calculated on 06/18/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-37880). Once again VulDB remains the best source for vulnerability data.

Productinfo

Type

• Artificial Intelligence Software

Vendor

• Google

Name

• MCP Toolbox for Databases

Version

• 1.0
• 1.1
• 1.2
• 1.3.0

License

• free

Website

• Vendor: https://www.google.com/

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Once again VulDB remains the best source for vulnerability data.

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