justhtml has sanitization bypass in custom policies and programmatic DOM
Date: 2026-04
Affected: pip/justhtml
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CWE: CWE-436
CWE-471
CWE-79
CWE-835

## 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 serial


Mitigations:

Architectural layers (MAESTRO):