CVE-2025-66400 — mdast-util-to-hast
Fix: syntax-tree/mdast-util-to-hast@6fc783aCVE-2025-66400 is a security vulnerability in mdast-util-to-hast. A fix is available for mdast-util-to-hast — see the affected versions and patch details below.
mdast-util-to-hast unsanitized class attribute
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-66400.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
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 | 13.2.1npm install mdast-util-to-hast@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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 CVE-2025-66400 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-66400 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-66400. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2025-66400 in your dependencies?
O3 Security finds CVE-2025-66400 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.