CVE-2026-63466 Unleash: Global Mustache.escape override disables HTML escaping process-wide, enabling Slack/Teams link-injection via unrestricted username
Attackers can inject malicious links into Slack or Teams notifications by exploiting disabled HTML escaping in Unleash feature events.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-63466?
CVE-2026-63466 is a medium-severity cross-site scripting vulnerability in unleash, published on 21 Aug 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
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.
Weakness: CWE-116
Which versions are affected by CVE-2026-63466?
2 open source packages ship an affected version, including unleash-server.
| Package | Affected | Fixed in |
|---|---|---|
unleash-server npm | All versions | 8.0.3 |
unleash-server npm | All versions | 8.0.3 |
How do I fix CVE-2026-63466?
Upgrade unleash-server to 8.0.3 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-63466 being exploited?
There is no known exploitation of CVE-2026-63466 in the wild and no public exploit code.
Sources
Also known as GHSA-w4mq-xh27-6xpx, MINI-mp52-g68v-c74g.