GHSA-pq4g-8rxf-4g29 - Crass: Incorrect tokenization of bad URLs enables CSS sanitizer bypass

## 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 7.0.0): - In `Sanitize::CSS.properties` and `style` attributes, a bad URL inside a `[...]` or `(...)` block in the value of an allowlisted property can smuggle arbitrary declarations, such as `position: fixed` or a `background-image` URL with a disallowed protocol. - In `Sanitize::CSS.stylesheet` and `<style>` elements, a bad URL in a selector can smuggle arbitrary style rules and at-rules, including `@import`. - **Loofah** (`Loofah::HTML5::Scrub.scrub_css`, used for `style` attributes), and t