GHSA-4fh9-h7wg-q85m is a medium-severity (CVSS 5.3) Improper Input Validation vulnerability in mdast-util-to-hast. O3 Security confirms whether GHSA-4fh9-h7wg-q85m is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
mdast-util-to-hast has unsanitized class attribute
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.
mdast-util-to-hastnpmDescription
Impact
Multiple (unprefixed) classnames could be added in markdown source by using character references.
This could make rendered user supplied markdown code elements appear like the rest of the page.
The following markdown:
```js xss
```
Would create <pre><code class="language-js xss"></code></pre>
If your page then applied .xss classes (or listeners in JS), those apply to this element.
For more info see https://github.com/ChALkeR/notes/blob/master/Improper-markup-sanitization.md#unsanitized-class-attribute
Patches
The bug was patched. When using regular semver, run npm install. For exact ranges, make sure to use 13.2.1.
Workarounds
Update.
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | mdast-util-to-hast | ≥ 13.0.0&&< 13.2.1 | 13.2.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mdast-util-to-hast. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update mdast-util-to-hast to 13.2.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4fh9-h7wg-q85m is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-4fh9-h7wg-q85m is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-4fh9-h7wg-q85m. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-4fh9-h7wg-q85m in your dependencies?
O3 detects GHSA-4fh9-h7wg-q85m across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.