PostCSS has XSS via Unescaped </style> in its CSS Stringify Output

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. Versions prior to 8.5.10 do 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. Version 8.5.10 fixes the issue.

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. Versions prior to 8.5.10 do 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. Version 8.5.10 fixes the issue.

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. Versions prior to 8.5.10 do 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. Version 8.5.10 fixes the issue.

# 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 ```javascript 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> executes ``` **Tested 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>: true ``` ## Impact 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: ```javascript output = output.replace(/<\/(style)/gi, '<\\/$1'); ``` ## Credits Discovered and reported by [Sunil Kumar](https://tharvid.in) ([@TharVid](https://github.com/TharVid))