CVE-2026-6827

## Summary `justhtml` `1.17.0` fixes multiple security issues in sanitization, serialization, and programmatic DOM handling. Most of these issues affected advanced or custom configurations rather than the default safe path. ## Affected versions - `justhtml` `<= 1.16.0` ## Fixed version - `justhtml` `1.17.0` released on April 19, 2026 ## Impact ### Custom SVG / MathML sanitization policies Custom policies that preserved foreign namespaces could allow dangerous content to survive sanitization, including: - active HTML integration points such as SVG `<foreignObject>`, MathML `<annotation-xml encoding="text/html">`, SVG `<title>` / `<desc>`, and MathML text integration points - mutation-XSS parser-differential payloads that looked inert in memory but became active HTML after reparse - SVG `filter="url(...)"` attributes that could trigger external fetches These issues affected: - `JustHTML(..., sanitize=True)` with custom foreign-namespace policies - `sanitize()` / `sanitize_dom()` - low-level terminal `Sanitize(...)` transform execution ### Preserved `<style>` handling Constructor-time sanitization and explicit `Sanitize(...)` transforms did not fully match `sanitize()` / `sanitize_dom()` when custom policies preserved `<style>`. That could leave resource-loading CSS such as `@import` or `background-image:url(...)` in sanitized output from HTML string input. ### Programmatic DOM serialization Programmatic `script`, `style`, and `Comment(...)` nodes could still serialize into active markup in some edge cases. This could affect applications that build or mutate DOM trees directly before calling `to_html()` or `to_markdown(html_passthrough=True)`. ### Cache mutation and DOM cycle handling Two lower-severity hardening fixes were included: - compiled sanitize-pipeline caches could be mutated after warming and weaken later sanitization - parent/child cycles in programmatic DOM trees could cause infinite loops in operations such as `to_html()` and `sanitize_dom()` ## Default configuration Most of the issues above did **not** affect ordinary parsed HTML with the default `JustHTML(..., sanitize=True)` configuration. The main risk areas were: - custom policies that preserve SVG or MathML - custom policies that preserve `<style>` - programmatic DOM construction or mutation - low-level direct sanitizer/transform APIs ## Recommended action Upgrade to `justhtml` `1.17.0`. If users cannot upgrade immediately: - avoid preserving SVG or MathML for untrusted input - avoid preserving `<style>` for untrusted input - avoid mutating programmatic DOM trees with untrusted `script`, `style`, or comment content - avoid mutating warmed policy internals or sanitizer caches ## Credit Discovered during an internal security review of `justhtml`.