CVE-2026-11718 googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)

Critical 9.3 Published 18 Jun 2026 Updated 18 Jun 2026

Attackers can bypass authentication by using tokens from unauthorized identity providers due to missing issuer validation in the MCP Toolbox library.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-11718?

CVE-2026-11718 is a critical authentication bypass vulnerability in MCP Toolbox for Databases (googleapis/mcp-toolbox), published on 18 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-287

Which versions are affected by CVE-2026-11718?

3 open source packages ship an affected version, including github.com/googleapis/mcp-toolbox. It affects 1 product.

Package	Affected	Fixed in
`github.com/googleapis/mcp-toolbox` Go	All versions	`1.4.0`
`github.com/googleapis/mcp-toolbox` Go	All versions	`1.4.0`
`github.com/googleapis/mcp-toolbox` Go	All versions	`1.4.0`

Products include Google Mcp Toolbox for Databases.

How do I fix CVE-2026-11718?

Upgrade github.com/googleapis/mcp-toolbox to 1.4.0 or later.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2026-11718 being exploited?

There is no known exploitation of CVE-2026-11718 in the wild and no public exploit code.

Sources

Also known as GHSA-wcpr-6g7x-p44r, GO-2026-5706.