GHSA-r758-8hxw-4845 — justhtml
Fix: EmilStenstrom/justhtml@8adc4dfGHSA-r758-8hxw-4845 is a security vulnerability in justhtml. A fix is available for justhtml — see the affected versions and patch details below.
justhtml: Mutation XSS with custom foreign-namespace sanitization policies
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-r758-8hxw-4845.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
justhtmlReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
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 | 1.14.0pip install --upgrade 'justhtml==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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-r758-8hxw-4845 can be triaged on real exposure rather than presence alone.
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 Security finds GHSA-r758-8hxw-4845 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.