# 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.

- **Severity:** Medium 4.1 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:N/I:L/A:N`)
- **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:** Yes
- **Published:** 21 Aug 2026
- **Updated:** 21 Aug 2026
- **Also known as:** GHSA-w4mq-xh27-6xpx, MINI-mp52-g68v-c74g

## 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 (Improper Encoding or Escaping of Output)

## Which versions are affected by CVE-2026-63466?

2 open source packages ship an affected version, including unleash-server.


| Package | Ecosystem | 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.

## Is CVE-2026-63466 being exploited?

There is no known exploitation of CVE-2026-63466 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63466 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-63466 in the wild and no public exploit code.

Affected packages and fixed versions:
- unleash-server (npm): affected all versions; fixed in 8.0.3
- unleash-server (npm): affected all versions; fixed in 8.0.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63466/ (Markdown: https://insights.sbomify.com/CVE-2026-63466/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63466
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63466
- OSV: https://osv.dev/vulnerability/CVE-2026-63466

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63466/
Summaries are generated and may contain mistakes; the official description above is the record.