---
title: "CVE-2026-11718: googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)"
url: https://erroneo.us/security/cve/CVE-2026-11718/
description: "CVE-2026-11718 (github.com/googleapis/mcp-toolbox (Go)): An authentication bypass vulnerability exists in the generic opaque token validation path…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-06-18
date_modified: 2026-06-19
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-11718"
kev: false
cvss: 9.3
epss: 0.00185
license: All rights reserved. Quote with attribution and a link.
---

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

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.2% , percentile 8: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 8% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes github.com/googleapis/mcp-toolbox (Go) < 1.4.0, fixed in 1.4.0 . Check your lockfile, including transitive dependencies.

## What it is

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

- Go `github.com/googleapis/mcp-toolbox` < 1.4.0 → fixed: 1.4.0

## Affected products (CVE record)

- Google MCP Toolbox for Databases (googleapis/mcp-toolbox): 1.0.0 <= 1.3.0

## Severity

- Google CVSS 4.0: 9.3 `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`
- GitHub CVSS 4.0: 9.3 `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`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.18% probability of exploitation activity in the next 30 days; percentile 8.

## Sources

- [GitHub Advisory GHSA-wcpr-6g7x-p44r](https://github.com/advisories/GHSA-wcpr-6g7x-p44r)
- [CVE.org record CVE-2026-11718](https://www.cve.org/CVERecord?id=CVE-2026-11718)
- [NVD entry CVE-2026-11718](https://nvd.nist.gov/vuln/detail/CVE-2026-11718)
- [github.com](https://github.com/googleapis/mcp-toolbox/pull/3360)

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Source page: https://erroneo.us/security/cve/CVE-2026-11718/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt