Vulnerability · Published Jun 18, 2026 · Assigned by Google
CVE-2026-11718
Critical 9.3
In plain English
A critical-severity improper authentication in MCP Toolbox for Databases (googleapis/mcp-toolbox) from Google. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 8% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Go | github.com/googleapis/mcp-toolbox | 1.4.0 | GHSA-wcpr-6g7x-p44r |
| Go | github.com/googleapis/mcp-toolbox | 1.4.0 | GO-2026-5706 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| MCP Toolbox for Databases (googleapis/mcp-toolbox) | 1.0.0 – through 1.3.0 |
Weakness: CWE-287 · Vector: 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