justhtml versions prior to 1.14.0 expose a mutation XSS when a custom SanitizationPolicy preserves foreign namespaces. An attacker can craft input that sanitizes to apparently safe markup, but when the browser or another parser re‑processes the output the payload is executed as script, enabling client‑side code execution. This flaw is a classic Injection/HTML Injection weakness, classified as CWE‑79.

The vulnerability affects the justhtml library developed by EmilStenstrom. Versions 1.13.0 and all earlier releases are impacted. The default safe configuration is immune; the flaw appears only when custom policies allow SVG, MathML, or raw‑text containers while preserving foreign namespaces.

The CVSS score of 5.1 indicates moderate severity. EPSS is not available, so the current exploitation probability is unknown, and the vulnerability is not listed in the CISA KEV catalog. Exploitation requires the attacker to supply crafted content to the sanitizer, which is feasible in applications that expose justhtml user input without upgrading. When custom policies are employed, the flaw can be leveraged; otherwise it remains mitigated by the default safe settings.