Summary
justhtml: Mutation XSS with custom foreign-namespace sanitization policies
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.
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.
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.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is GHSA-R758-8HXW-4845? GHSA-R758-8HXW-4845 is a low-severity cross-site scripting (XSS) vulnerability in justhtml (pip), affecting versions >= 1.13.0, < 1.14.0. It is fixed in 1.14.0. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of justhtml are affected by GHSA-R758-8HXW-4845? justhtml (pip) versions >= 1.13.0, < 1.14.0 is affected.
- Is there a fix for GHSA-R758-8HXW-4845? Yes. GHSA-R758-8HXW-4845 is fixed in 1.14.0. Upgrade to this version or later.
- Is GHSA-R758-8HXW-4845 exploitable, and should I be worried? Whether GHSA-R758-8HXW-4845 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-R758-8HXW-4845 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-R758-8HXW-4845? Upgrade
justhtmlto 1.14.0 or later.