GHSA-3949-f494-cm99 is a high-severity (CVSS 7.5) Cross-site Scripting (XSS) vulnerability in prismjs. O3 Security confirms whether GHSA-3949-f494-cm99 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Cross-site Scripting in Prism
Real-World Exposure
prismjsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Prism's Command line plugin can be used by attackers to achieve an XSS attack. The Command line plugin did not properly escape its output, leading to the input text being inserted into the DOM as HTML code.
Server-side usage of Prism is not impacted. Websites that do not use the Command Line plugin are also not impacted.
Patches
This bug has been fixed in v1.27.0.
Workarounds
Do not use the Command line plugin on untrusted inputs, or sanitized all code blocks (remove all HTML code text) from all code blocks that use the Command line plugin.
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | prismjs | ≥ 1.14.0&&< 1.27.0 | 1.27.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for prismjs. 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 prismjs to 1.27.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-3949-f494-cm99 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-3949-f494-cm99 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-3949-f494-cm99. 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-3949-f494-cm99 in your dependencies?
O3 detects GHSA-3949-f494-cm99 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.