GHSA-vrx2-77f2-ww34

## 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`.