# CVE-2026-63466: Unleash global Mustache.escape override disables HTML escaping in notifications

Source: https://junglewise.ai/threats/cve-2026-63466-unleash-global-mustache-escape-override-disables-html-escaping-in  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-21. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-63466 · Severity: medium · CVSS 4.1 · Published 2026-08-21

Unleash, a feature flag management platform, has a vulnerability in its notification system that disables HTML escaping process-wide when certain notification integrations (Slack, Teams, Webhooks, Datadog, New Relic) format events. An attacker with an Editor role can set a malicious username containing link-injection syntax, which then appears unescaped in outbound notifications to these platforms, enabling phishing attacks via clickable malicious links injected into trusted notification feeds.

Technologies: [unleash-server (npm)](https://junglewise.ai/threats/technologies/unleash-server). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-63466 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63466 being exploited in the wild.

**How severe is CVE-2026-63466?** Medium severity, with a CVSS score of 4.1, published 21 August 2026.

**Which products and versions are affected?** unleash-server (npm) < 8.0.3, SEMVER:introduced=0,fixed=8.0.3.

**Is there a fix for CVE-2026-63466?** Yes. unleash-server (npm): upgrade to 8.0.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63466 attacked?** It is attacked over the network, in a hosted service.

**Am I affected by CVE-2026-63466?** Check whether you run unleash-server (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- unleash-server (npm): upgrade to 8.0.3 or later

  ```sh
  npm install unleash-server@8.0.3
  ```


## Technical details

The vulnerability lies in `FeatureEventFormatterMd.format()` (src/lib/addons/feature-event-formatter-md.ts:355), which globally mutates `Mustache.escape = (text) => text;` to disable HTML escaping during template rendering. Since Mustache's `escape` function is a module-level singleton shared across the entire Node.js process, this single assignment permanently disables escaping for all downstream `Mustache.render()` calls in email-service, webhook, Datadog, and New Relic formatters until process restart. The attack vector requires an attacker-controlled username field (event.createdBy) to be interpolated into event templates via single-mustache syntax `{{user}}`. Usernames lack validation and can contain characters like `&lt;`, `&gt;`, and `|` that Slack/Teams use for link injection syntax (`&lt;url|label&gt;`). An Editor-level account holder (or self-registered user if public signup is enabled) can perform routine actions (toggle a flag), triggering event formatting that renders the malicious username unescaped into notification payloads sent to Slack, Teams, or custom webhooks. The fix is to pass an escape function via per-call render options (`RenderOptions.escape`) rather than mutating the global singleton. Fixed in version 8.0.3.

## Affected products

- Unleash unleash-server < 8.0.3

## Identifiers

- CVE-2026-63466
- GHSA-w4mq-xh27-6xpx

## Timeline

- 2026-08-21: disclosed: Published in GitHub Advisory Database
- 2026-08-21: patched: Fixed in v8.0.3

## References

- [https://api.github.com/users/iaohkut-from-NightWolf-Team](https://api.github.com/users/iaohkut-from-NightWolf-Team)
- [https://github.com/iaohkut-from-NightWolf-Team](https://github.com/iaohkut-from-NightWolf-Team)
- [https://api.github.com/users/iaohkut-from-NightWolf-Team/gists%7B/gist_id%7D](https://api.github.com/users/iaohkut-from-NightWolf-Team/gists%7B/gist_id%7D)
- [https://api.github.com/users/iaohkut-from-NightWolf-Team/repos](https://api.github.com/users/iaohkut-from-NightWolf-Team/repos)
- [https://avatars.githubusercontent.com/u/286391906?v=4](https://avatars.githubusercontent.com/u/286391906?v=4)
- [https://api.github.com/users/iaohkut-from-NightWolf-Team/events%7B/privacy%7D](https://api.github.com/users/iaohkut-from-NightWolf-Team/events%7B/privacy%7D)

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