CVE-2026-5388
CriticalCVSS 9.8Summary
justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers, HTML serialization, Markdown passthrough (html_passthrough=True), and several custom sanitization-policy edge cases. An attacker can bypass sanitization to inject active HTML and JavaScript, e.g., via encoded javascript: URLs, backslash-based relative URLs, markup-breaking names, raw </textarea> reintroduction, or preserved <style>/<meta http-equiv=refresh>/<base href> tags.
Risk Assessment
The organization may be exposed to XSS and other attacks if using helper APIs, programmatic DOM, html_passthrough=True, or custom policies. The default sanitize=True configuration is less vulnerable, but risk remains in advanced scenarios.
Recommendation
Update justhtml to version 1.15.0 or later. Avoid using html_passthrough=True and custom policies unless necessary, and always use default sanitization.
Original NVD description (English source)
justhtml before 1.15.0 contains multiple security issues in URL sanitization helpers (clean_url_value/clean_url_in_js_string), HTML serialization, Markdown passthrough (html_passthrough=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, html_passthrough=True, and custom policies/transform pipelines.