Vulnerability · Published Aug 21, 2026 · Assigned by GitHub_M
CVE-2026-63466Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username
Medium 4.1
In plain English
A medium-severity security flaw in unleash from Unleash. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Unleash is an open-source feature management platform. Prior to 8.0.3, FeatureEventFormatterMd.format in src/lib/addons/feature-event-formatter-md.ts assigns Mustache.escape to an identity function before rendering action and path templates. Because Mustache.escape is process-wide, the assignment disables escaping for subsequent Mustache.render calls in email-service.ts, webhook.ts, datadog.ts, and new-relic.ts. An editor-level user can place Slack or Microsoft Teams link syntax in an unrestricted username, trigger a feature event, and inject an attacker-labeled link into a trusted outbound notification channel, while other Mustache sinks remain unescaped until restart. This issue is fixed in version 8.0.3.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | unleash-server | 8.0.3 | GHSA-w4mq-xh27-6xpx |
Affected products (as reported)
Weakness: CWE-116 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N
References
- https://github.com/Unleash/unleash/security/advisories/GHSA-w4mq-xh27-6xpx x_refsource_CONFIRM
- https://github.com/Unleash/unleash/commit/002012cfdbedd2e9b7db9dc83b9f549f761db22e x_refsource_MISC
- https://github.com/Unleash/unleash/releases/tag/v8.0.3 x_refsource_MISC