Crass is a Ruby CSS parser that is compliant with the CSS Syntax Level 3 specification. Prior to 1.0.8, Crass incorrectly treats a closing parenthesis after an escaped backslash as escaped while consuming the remnants of a bad URL. Crass therefore keeps declarations, style rules, or at-rules inside a bad URL token even though a browser treats them as live CSS. A sanitizer that parses untrusted CSS with Crass and reserializes it without rejecting the bad URL can allow CSS used for UI redress, external resource loading, or CSS-based data exfiltration, but this issue does not enable script execution in modern browsers. This issue is fixed in version 1.0.8.

Affected: RubyGems: crass

## Summary

Crass's tokenizer didn't terminate a bad URL token at a `)` that followed an escaped backslash (`\\`). As a result, Crass consumed more input as part of the bad URL than a spec-compliant browser does. CSS sanitizers that parse untrusted CSS with Crass and re-emit it could be tricked into passing through declarations, style rules, or at-rules (including `@import`) that the sanitizer would otherwise have removed, but that a browser would then parse and apply.

## Details

While consuming the remnants of a bad URL (CSS Syntax Level 3, §4.3.15), Crass checked whether the *previously consumed* code point and the next code point formed a valid escape, in addition to checking the next two unconsumed code points. After an escaped backslash, the previously consumed code point is a backslash, so a following `)` was incorrectly treated as escaped and the bad URL token continued until the next unescaped `)` or the end of the input.

Browsers terminate the bad URL at that `)`, so everything Crass treated as part of the bad URL is live CSS to a browser.

This bug has existed in every released version of Crass.

## Impact

Crass by itself only mis-tokenizes input. The security impact is on CSS sanitizers that use Crass to make allow/deny decisions and then re-serialize the CSS. Exploitation requires a sanitizer that doesn't reject bad URL tokens wherever they occur. Confirmed examples (verified in Chromium against the sanitizers' output):

- **Sanitize** (all versions through 7…

Affected: Ubuntu:16.04:LTS: ruby-crass, Ubuntu:18.04:LTS: ruby-crass, Ubuntu:20.04:LTS: ruby-crass, Ubuntu:22.04:LTS: ruby-crass, Ubuntu:24.04:LTS: ruby-crass, Ubuntu:26.10: ruby-crass, Ubuntu:26.04:LTS: ruby-crass

Crass is a Ruby CSS parser that is compliant with the CSS Syntax Level 3 specification. Prior to 1.0.8, Crass incorrectly treats a closing parenthesis after an escaped backslash as escaped while consuming the remnants of a bad URL. Crass therefore keeps declarations, style rules, or at-rules inside a bad URL token even though a browser treats them as live CSS. A sanitizer that parses untrusted CSS with Crass and reserializes it without rejecting the bad URL can allow CSS used for UI redress, external resource loading, or CSS-based data exfiltration, but this issue does not enable script execution in modern browsers. This issue is fixed in version 1.0.8.