CVE-2026-11718: googleapis/mcp-toolbox Authentication Bypass Vulnerability

CRITICAL
Advisory Identifier: GHSA-wcpr-6g7x-p44r
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CWE-287
Published
Advisory Summary

CVE-2026-11718 is a CRITICAL authentication bypass vulnerability (CWE-287: Improper Authentication) affecting googleapis/mcp-toolbox libraries in the Go ecosystem. The issue involves authentication bypass vulnerability in the generic opaque token validation path (validateopaquetoken). Remediation is available by upgrading affected packages to version 1.4.0.

CVE-2026-11718 at a Glance

CVE Identifier CVE-2026-11718
GHSA Identifier GHSA-wcpr-6g7x-p44r
Severity CRITICAL
CVSS v3.1 Base Score N/A
CWE Classification CWE-287: Improper Authentication
Affected Product googleapis/mcp-toolbox
Package Ecosystem Go
Published Date
Last Updated

How to Fix CVE-2026-11718

To remediate this vulnerability, upgrade all affected googleapis/mcp-toolbox dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

github.com/googleapis/mcp-toolbox fixed: 1.4.0 
Upgrade github.com/googleapis/mcp-toolbox to 1.4.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for github.com/googleapis/mcp-toolbox

How to Check if You Are Affected

To determine whether your application is affected by CVE-2026-11718, inspect your project dependencies for googleapis/mcp-toolbox:

inspect dependencies for github.com/googleapis/mcp-toolbox

Analyze the terminal output to locate any version of googleapis/mcp-toolbox in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.4.0), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-287: Improper Authentication, where when an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. In googleapis/mcp-toolbox, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

CVE-2026-11718 Affected googleapis/mcp-toolbox Versions

1 component(s)
Fixed Versions:
1.4.0
Product / Package Ecosystem Affected Versions Fixed Version
github.com/googleapis/mcp-toolbox
golang/github.com/googleapis/mcp-toolbox
Go
< 1.4.0
1.4.0

References

NVD — CVE-2026-11718 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-11718
googleapis/mcp-toolbox Source Repository REPOSITORY
https://github.com/googleapis/mcp-toolbox/pull/3360
googleapis/mcp-toolbox Source Repository REPOSITORY
https://github.com/googleapis/mcp-toolbox

Frequently Asked Questions

What is CVE-2026-11718?
CVE-2026-11718 is a security vulnerability in googleapis/mcp-toolbox involving authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken) under CWE-287: Improper Authentication.
Which googleapis/mcp-toolbox versions are affected?
Affected versions include releases prior to 1.4.0 across affected packages: github.com/googleapis/mcp-toolbox.
What version fixes CVE-2026-11718?
Upgrading to 1.4.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/googleapis/mcp-toolbox affected?
Yes, github.com/googleapis/mcp-toolbox is affected in releases prior to 1.4.0. Upgrade your dependency to 1.4.0 or later.
What CWE is associated with CVE-2026-11718?
CVE-2026-11718 is classified under CWE-287: Improper Authentication (When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.).
How can I check Go dependencies for CVE-2026-11718?
Run the dependency tree command: 'inspect dependencies for github.com/googleapis/mcp-toolbox'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-wcpr-6g7x-p44r",
  "modified" : "2026-06-19T16:59:25Z",
  "published" : "2026-06-18T15:32:01Z",
  "aliases" : [ "CVE-2026-11718" ],
  "summary" : "googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)",
  "details" : "An authentication bypass vulnerability exists in the generic opaque token validation path (validateOpaqueToken) of googleapis/mcp-toolbox.\n\nWhen 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.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/googleapis/mcp-toolbox"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.4.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-11718"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/googleapis/mcp-toolbox/pull/3360"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/googleapis/mcp-toolbox"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-287" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T16:59:25Z",
    "nvd_published_at" : "2026-06-18T14:17:20Z"
  }
}