Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.1 · CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N

EPSS

• CVE-2026-63466: 0.32% estimated exploitation activity in the next 30 days; percentile 24.93 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Vulnerability Details File: src/lib/addons/feature-event-formatter-md.ts Line: 355 (in v8.0.1; format() method) Root Cause FeatureEventFormatterMd.format() does: mustache (pinned ^4.2.0, confirmed installed 4.2.0) keeps escape as a module-level singleton (mustache.js: mustache.escape = escapeHtml;), read by every Mustache.render() call in the process unless a per-call config.escape override is passed (var escape = this.getConfigEscape(config) || mustache.escape;). Node's module cache guarantees every import Mustache from 'mustache' in the process — feature-event-formatter-md.ts, email-service.ts, webhook.ts, datadog.ts, new-relic.ts — shares the same object instance. This assignment therefore permanently disables HTML escaping for every other Mustache.render() call in the same Node process (including email-service.ts templates) from the moment any single notification addon (Webhook, Slack legacy, Microsoft Teams, Datadog, New Relic) first formats any event, for the remaining lifetime…

Published by the source

21 Aug 2026, 19:14 UTC

Last modified by the source

21 Aug 2026, 19:14 UTC

Source last checked

26 Oct 2026, 13:01 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-w4mq-xh27-6xpx

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# Patch Next advisory brief: CVE-2026-63466

Advisory: Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username
IDs: CVE-2026-63466, GHSA-w4mq-xh27-6xpx
Record: https://patchnext.com/advisories/cve-2026-63466 (revision 1, source checked 26 Oct 2026, 13:01 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: unleash-server 8.0.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm unleash-server: affected < 8.0.3; fixed in 8.0.3

## Suggested inspection (read-only)
- `npm ls unleash-server --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on unleash-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-w4mq-xh27-6xpx (GitHub-reviewed advisory GHSA-w4mq-xh27-6xpx lists affected ranges: npm unleash-server >=0 <8.0.3)

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