CVE-2026-11717: googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• 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.

From the GitHub advisory

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.

Weakness type: CWE-287.

Product: github.com/googleapis/mcp-toolbox (Go) · Assigned by: Google · Published: 2026-06-18

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• 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.

Timeline

• 2026-06-18 — CVE record last updated
• 2026-06-18 — CVE record published by Google
• 2026-06-18 — GitHub-reviewed advisory GHSA-8fcc-w5hv-4gxv published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-8fcc-w5hv-4gxv github.com
2. CVE.org record CVE-2026-11717 cve.org
3. NVD entry CVE-2026-11717 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com

Critical Go advisories · Today’s Security Digest