GHSA-c4rq-3m3g-8wgx

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

Credit

Vector 1 was responsibly reported by @colby-swandale. Vectors 2 and 3 were discovered by @flavorjones during the response to the original report.