GHSA-c4rq-3m3g-8wgx

Nokogiri CSS selector tokenizer has regular expression backtracking

## Summary Nokogiri's CSS selector tokenizer contains regular expressions whose construction may result in exponential regex backtracking on adversarial selectors. Three ReDoS vectors are addressed in this release: 1. String-literal tokenization on certain unterminated quoted-string input. 2. String-literal tokenization on a separate class of hex-escape-rich input. 3. Identifier tokenization on hex-escape-rich input. The public CSS selector methods that funnel through the affected tokenizer are `Nokogiri::CSS.xpath_for`, `Node#css`, `Node#at_css`, `Searchable#search`, and `CSS::Parser#parse`. ## Mitigation Upgrade to Nokogiri `>= 1.19.3`. If users are unable to upgrade, two options are available: - Avoid the use of attacker-controlled text in CSS selectors. Applications that only pass developer-authored selectors to Nokogiri are not directly exposed. - Set global `Regexp.timeout` (Ruby 3.2+, JRuby 9.4+) to bound parse time. ## Severity The Nokogiri maintainers have evaluated this as **High Severity** (CVSS 7.5, `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`). An attacker able to inject user-supplied text into a CSS selector parse method can cause exponential backtracking, resulting in a potential denial of service. ## Resources - [CWE-1333: Inefficient Regular Expression Complexity](https://cwe.mitre.org/data/definitions/1333.html) ## Credit Vector 1 was responsibly reported by @colby-swandale. Vectors 2 and 3 were discovered by @flavorjones during the response to the original report.