CVE-2026-5388

justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (cleanurlvalue/cleanurlinjsstring), HTML serialization, Markdown passthrough (htmlpassthrough=True), and several custom sanitization-policy edge cases. Depending on configuration, an attacker can bypass sanitization to inject active HTML and JavaScript — for example via encoded javascript: URLs, backslash-based relative URLs resolved as remote hosts, markup-breaking programmatic element/attribute names or HTML comments, raw </textarea> reintroduction through Markdown passthrough, or preserved <style>/<meta http-equiv=refresh>/<base href> tags in custom policies. Most custom-policy issues do not affect the default sanitize=True configuration; they primarily affect helper APIs, programmatic DOM construction, htmlpassthrough=True, and custom policies/transform pipelines.