---
title: "CVE-2026-11717: googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)"
url: https://erroneo.us/security/cve/CVE-2026-11717/
description: "CVE-2026-11717 (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-2026-11717"
kev: false
cvss: 9.3
epss: 0.00176
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-11717: 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 7: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 7% 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 verifying an unparsed opaque token via an OAuth 2.0 introspection endpoint (RFC 7662), the toolbox decodes the response into an introspectResp struct where the Active field is declared as a pointer to a boolean (bool). The code only explicitly rejects a token if the response contains a populated active field set to false (if introspectResp.Active != nil && !introspectResp.Active). If an introspection endpoint responds with a payload that completely omits the mandatory active key, the internal variable remains nil, causing the conditional check to short-circuit. As a result, Toolbox accepts authorization tokens missing the "active" field, granting access to protected tools and underlying data sources.

## 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 7.

## Sources

- [GitHub Advisory GHSA-8fcc-w5hv-4gxv](https://github.com/advisories/GHSA-8fcc-w5hv-4gxv)
- [CVE.org record CVE-2026-11717](https://www.cve.org/CVERecord?id=CVE-2026-11717)
- [NVD entry CVE-2026-11717](https://nvd.nist.gov/vuln/detail/CVE-2026-11717)
- [github.com](https://github.com/googleapis/mcp-toolbox/pull/3341)

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Source page: https://erroneo.us/security/cve/CVE-2026-11717/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt