How severe is GHSA-8fcc-w5hv-4gxv?

No CVSS score has been assigned to GHSA-8fcc-w5hv-4gxv yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-8fcc-w5hv-4gxv?

GHSA-8fcc-w5hv-4gxv affects the following packages: github.com/googleapis/mcp-toolbox (Go). Ecosystems affected: Go.

How do I fix GHSA-8fcc-w5hv-4gxv?

Update github.com/googleapis/mcp-toolbox to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-8fcc-w5hv-4gxv is resolved across your whole dependency graph.

How do I detect GHSA-8fcc-w5hv-4gxv in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/googleapis/mcp-toolbox, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate GHSA-8fcc-w5hv-4gxv if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is GHSA-8fcc-w5hv-4gxv actively exploited in the wild?

No public exploit code has been indexed for GHSA-8fcc-w5hv-4gxv yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for GHSA-8fcc-w5hv-4gxv?

GHSA-8fcc-w5hv-4gxv has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 7th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is GHSA-8fcc-w5hv-4gxv?

GHSA-8fcc-w5hv-4gxv is classified as Improper Authentication (CWE-287). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-8fcc-w5hv-4gxv published, and has it been updated?

GHSA-8fcc-w5hv-4gxv was published on June 18, 2026 and was last updated on June 25, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

googleapis/mcp-toolbox: authentication bypass vulnerability in the generic opaque token validation path (validateOpaqueToken)GHSA-8fcc-w5hv-4gxv

Fix: googleapis/mcp-toolbox#3341

GHSA-8fcc-w5hv-4gxv is a Improper Authentication vulnerability in github.com/googleapis/mcp-toolbox. A fix is available for github.com/googleapis/mcp-toolbox — see the affected versions and patch details below.

Also known asCVE-2026-11717GO-2026-5251

Published

Jun 18, 2026

Updated

Jun 25, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 31, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-8fcc-w5hv-4gxv.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs7th percentile — riskier than 7% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

1 pkg affected

🐹github.com/googleapis/mcp-toolbox

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

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

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/googleapis/mcp-toolbox`	all versions	1.4.0`go get github.com/googleapis/mcp-toolbox@v1.4.0`

Affected Products

1 product · 1 configurations

Application

mcp toolbox for databasesgoogle

≥ 1.0.0 && ≤ 1.3.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/googleapis/mcp-toolbox, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/googleapis/mcp-toolbox to 1.4.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-8fcc-w5hv-4gxv is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

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

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