PostCSS has XSS via Unescaped </style> in its CSS Stringify Output
PostCSS: XSS via Unescaped </style> in CSS Stringify Output
Summary
PostCSS v8.5.5 (latest) does not escape </style> sequences when stringifying CSS ASTs. When user-submitted CSS is parsed and re-stringified for embedding in HTML <style> tags, </style> in CSS values breaks out of the style context, enabling XSS.
Proof of Concept
const postcss = require('postcss');
// Parse user CSS and re-stringify for page embedding
const userCSS = 'body { content: "</style><script>alert(1)</script><style>"; }';
const ast = postcss.parse(userCSS);
const output = ast.toResult().css;
const html = `<style>${output}</style>`;
console.log(html);
// <style>body { content: "</style><script>alert(1)</script><style>"; }</style>
//
// Browser: </style> closes the style tag, <script> executesTested output (Node.js v22, postcss v8.5.5):
Input: body { content: "</style><script>alert(1)</script><style>"; }
Output: body { content: "</style><script>alert(1)</script><style>"; }
Contains </style>: trueImpact
Impact non-bundler use cases since bundlers for XSS on their own. Requires some PostCSS plugin to have malware code, which can inject XSS to website.
Suggested Fix
Escape </style in all stringified output values:
output = output.replace(/<\/(style)/gi, '<\\/$1');Credits
Discovered and reported by Sunil Kumar (@TharVid)
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | postcss | < 8.5.10 | 8.5.10 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N- Weakness (CWE)
CWE-79- Platform
- npm
- Package
postcss- Disclosed
- 24.04.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-qx2v-qp2m-jg93 - CC BY 4.0
Vulnerability history: postcss
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 7
- First → latest
- 05.2021 08.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 2
- Medium 5
Disclosures per year
- 2021: 1
- 2022: 1
- 2023: 1
- 2024: 0
- 2025: 0
- 2026: 4
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PostCSS has XSS via Unescaped </style> in its CSS Stringify Output this entry
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