Multiple security fixes in justhtml
Date: 2026-04
Affected: pip/justhtml
Attack vector: xss
Category: vulnerability-disclosure
Severity: Low
CWE: CWE-178
CWE-436
CWE-471
CWE-79

## Summary `justhtml` `1.16.0` fixes multiple security issues in sanitization, serialization, and programmatic DOM handling. Most of these issues affected one of these advanced paths rather than ordinary parsed HTML with the default safe settings: - programmatic DOM input to `sanitize()` or `sanitize_dom()` - reused or mutated sanitization policy objects - custom policies that preserve foreign namespaces such as SVG or MathML ## Affected versions - `justhtml` `<= 1.15.0` ## Fixed version - `justhtml` `1.16.0` released on April 12, 2026 ## Impact ### Policy reuse and mutation Nested mutation of sanitization policy internals could weaken later sanitization by leaving stale compiled sanitizers active, or by mutating exported default policy internals process-wide. ### In-memory sanitization gaps Programmatic DOM sanitization could miss dangerous mixed-case tag names such as `ScRiPt` or `StYlE`, and custom `drop_content_tags` values such as `{"SCRIPT"}` could silently fail to drop dangerous subtrees. ### Serialization injection Crafted programmatic doctype names could serialize into active markup before the document body. ### Foreign-namespace policy bypasses Custom policies that preserve SVG or MathML could allow active SVG features to survive sanitization, including: - animation elements such as `<set>` and `<animate>` that mutate already-sanitized attributes after sanitization - presentation attributes such as `fill`, `clip-path`, `mask`, `marker-start`, and `curso


Mitigations:

Architectural layers (MAESTRO):