justhtml: Mutation XSS with custom foreign-namespace sanitization policies

Summary

A parser-differential / mutation XSS issue was found in justhtml when using a custom sanitization policy that preserves foreign namespaces such as SVG or MathML.

Under these custom settings, specially crafted input could sanitize into HTML that looked safe at first, but became unsafe when parsed again by a browser or another HTML parser.

Impact

This issue does not affect the default safe configuration.

You may be affected if you use a custom SanitizationPolicy with settings like:

• drop_foreign_namespaces=False
• allowlisted foreign elements such as MathML or SVG
• allowlisted raw-text containers such as <style>

In that case, an attacker could inject markup that survives sanitization and turns into active HTML after re-parsing.

Affected versions

• justhtml <= 1.13.0

Fixed version

• Fixed in 1.14.0

Workarounds

Until you upgrade:

• keep drop_foreign_namespaces=True
• avoid allowlisting foreign namespaces for untrusted input
• avoid allowlisting raw-text containers such as <style> in custom policies

Notes

The default JustHTML(..., sanitize=True) behavior was not found to be vulnerable in this issue.

Credit

Discovered by JustHTML author during a LLM-based security review of justhtml.