justhtml before 1.18.0 contains multiple low-severity denial-of-service issues in CSS selector handling and linkification. Applications that evaluate attacker-controlled selector strings (via query(), matches(), or selector-based transforms), run selector matching over very large untrusted documents, construct DOM trees from untrusted structure, or enable linkification over attacker-controlled text may consume disproportionate CPU or memory. Triggers include oversized selectors, large selector lists, oversized compound selectors, long combinator chains, deeply nested functional pseudo-classes, repeated token/positional matching, cyclic DOM graphs causing non-terminating traversal, and punctuation-heavy or trailing-bracket linkification input. These are availability-only concerns and do not by themselves allow script execution, data disclosure, or sanitizer bypass. Default JustHTML(sanitize=True) usage is not expected to be exposed, since selectors are normally supplied by application code.

CVE-2026-4671 affects justhtml versions before 1.18.0, describing multiple low-severity denial-of-service vulnerabilities in CSS selector handling and linkification. Affected applications include those evaluating attacker-controlled selector strings via query(), matches(), or selector-based transforms, processing large untrusted documents, constructing DOM trees from untrusted input, or enabling linkification over attacker-controlled text. Attack triggers include oversized selectors, large selector lists, long combinator chains, deeply nested functional pseudo-classes, cyclic DOM graphs, and punctuation-heavy linkification input. Exploitation may cause disproportionate CPU or memory consumption. These issues are strictly availability-only concerns and do not enable script execution, data disclosure, or sanitizer bypass. The default JustHTML(sanitize=True) configuration is generally not exposed to these issues since selectors are normally supplied by application code. The fix is available in justhtml version 1.18.0.