{"id":"GHSA-pq4g-8rxf-4g29","summary":"Crass: Incorrect tokenization of bad URLs enables CSS sanitizer bypass","details":"## Summary\n\nCrass'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.\n\n## Details\n\nWhile 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.\n\nBrowsers terminate the bad URL at that `)`, so everything Crass treated as part of the bad URL is live CSS to a browser.\n\nThis bug has existed in every released version of Crass.\n\n## Impact\n\nCrass 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):\n\n- **Sanitize** (all versions through 7.0.0):\n  - 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.\n  - In `Sanitize::CSS.stylesheet` and `\u003cstyle\u003e` elements, a bad URL in a selector can smuggle arbitrary style rules and at-rules, including `@import`.\n- **Loofah** (`Loofah::HTML5::Scrub.scrub_css`, used for `style` attributes), and therefore **rails-html-sanitizer** (`sanitize_css`, and `sanitize` when `style` is a permitted attribute): a bad URL inside an allowlisted function such as `calc()` or `rgb()` can smuggle declarations Loofah normally strips, such as `position: fixed` and `url()` values.\n\nSmuggled CSS can be used for UI redress (clickjacking or phishing overlays), loading attacker-controlled external resources, and, where rules or `@import` can be smuggled, CSS-based data exfiltration. This vulnerability doesn't allow script execution in modern browsers.\n\n## Patches\n\nFixed in Crass 1.0.8. Users of Sanitize, Loofah, rails-html-sanitizer, or any other library that uses Crass to sanitize CSS should upgrade Crass to 1.0.8 or later.\n\n## Workarounds\n\nThere are no practical workarounds other than upgrading. Rejecting any CSS input that contains the character sequence `\\\\` would block the known trigger, but this is a blunt measure and may reject legitimate CSS.\n\n## References\n\n- CSS Syntax Module Level 3, \"Consume the remnants of a bad url\": https://drafts.csswg.org/css-syntax-3/#consume-remnants-of-bad-url","aliases":["CVE-2026-116695"],"modified":"2026-10-28T23:00:04.150123033Z","published":"2026-10-28T22:42:53Z","database_specific":{"nvd_published_at":"2026-10-27T21:18:05Z","cwe_ids":["CWE-436"],"severity":"MODERATE","github_reviewed":true,"github_reviewed_at":"2026-10-28T22:42:53Z"},"references":[{"type":"WEB","url":"https://github.com/rgrove/crass/security/advisories/GHSA-pq4g-8rxf-4g29"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-116695"},{"type":"WEB","url":"https://github.com/rgrove/crass/commit/689fc45c632fdf3803b2f4e844ffc9cca3b08f34"},{"type":"PACKAGE","url":"https://github.com/rgrove/crass"},{"type":"WEB","url":"https://github.com/rgrove/crass/releases/tag/v1.0.8"},{"type":"WEB","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/crass/GHSA-pq4g-8rxf-4g29.yml"}],"affected":[{"package":{"name":"crass","ecosystem":"RubyGems","purl":"pkg:gem/crass"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.0.8"}]}],"versions":["0.0.1","0.0.2","0.1.0","0.2.0","0.2.1","1.0.0","1.0.1","1.0.2","1.0.3","1.0.4","1.0.5","1.0.6","1.0.7"],"database_specific":{"last_known_affected_version_range":"\u003c= 1.0.7","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/10/GHSA-pq4g-8rxf-4g29/GHSA-pq4g-8rxf-4g29.json"}}],"schema_version":"1.9.2","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"}]}