GHSA-r758-8hxw-4845
justhtml: Mutation XSS with custom foreign-namespace sanitization policies
Blast Radius
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Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | justhtml | ≥ 1.13.0&&< 1.14.0 | 1.14.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for justhtml. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update justhtml to 1.14.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-r758-8hxw-4845 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-r758-8hxw-4845 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-r758-8hxw-4845. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-r758-8hxw-4845 in your dependencies?
O3 detects GHSA-r758-8hxw-4845 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.