Nokogiri versions before 1.19.3 contain regular expression denial of service vulnerabilities in the CSS selector tokenizer affecting string-literal and identifier tokenization. Attackers can inject adversarial CSS selectors into methods like Node#css, Node#at_css, and Searchable#search to cause exponential regex backtracking and denial of service.

Nokogiri versions prior to 1.19.3 are vulnerable to Regular Expression Denial of Service (ReDoS) in the CSS selector tokenizer. The vulnerability affects string-literal and identifier tokenization within the library. Attackers can exploit this by injecting adversarial CSS selectors into methods such as Node#css, Node#at_css, and Searchable#search. This causes exponential regex backtracking, leading to denial of service conditions. The vulnerability is tracked as CVE-2026-79770 and has been patched in Nokogiri 1.19.3. Users of affected versions should upgrade immediately to mitigate the risk. The issue is documented across NVD, GitHub Security Advisories, and VulnCheck.