JustHTML Affected by Mutation XSS via Literal Text Serialization in Raw Text Elements (style/script)
Date: 2026-03
Affected: pip/justhtml
Attack vector: xss
Category: vulnerability-disclosure
Severity: Medium
CWE: CWE-79

## Summary Sanitized DOM trees can be unsafe to serialize when a custom policy allows raw-text elements such as `<style>` or `<script>`. The issue affects DOM trees that are constructed or modified programmatically and then passed through `sanitize_dom()` with a policy that keeps these elements. Text nodes inside `<style>` and `<script>` are serialized literally, so attacker-controlled text containing the matching closing tag sequence can break out of the raw-text context and inject HTML into the serialized output. The default sanitization policy is not affected because it drops the contents of `style` and `script`. ## Details The root cause is in HTML serialization of raw-text elements. In serialize.py, text children of `script` and `style` are emitted verbatim: ```python _LITERAL_TEXT_SERIALIZATION_ELEMENTS = frozenset({"script", "style"}) def _serialize_text_for_parent(text: str | None, parent_name: str | None) -> str: if not text: return "" if parent_name in _LITERAL_TEXT_SERIALIZATION_ELEMENTS: return text return _escape_text(text)


Mitigations:

Architectural layers (MAESTRO):