---
id: CVE-2026-5751
aliases:
  - GHSA-r758-8hxw-4845
title: 'justhtml: Mutation XSS with custom foreign-namespace sanitization policies'
summary: 'justhtml: Mutation XSS with custom foreign-namespace sanitization policies'
severity: low
vendor: justhtml
product: justhtml
ecosystem: pip
affected:
  - 'justhtml >= 1.13.0, < 1.14.0'
patched:
  - justhtml 1.14.0
published: '2026-04-08'
updated: '2026-08-24'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-r758-8hxw-4845'
references:
  - url: >-
      https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845
  - url: >-
      https://github.com/EmilStenstrom/justhtml/commit/8adc4dfe31723dc522f44338f9bd98381c92b076
  - url: 'https://github.com/EmilStenstrom/justhtml'
  - url: 'https://github.com/EmilStenstrom/justhtml/releases/tag/v1.14.0'
tags:
  - osv
  - pip
epss: 0.00263
epssPercentile: 0.16148
ingestedAt: '2026-08-24T19:25:44.289Z'
---

## Overview

## 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

- `justhtml >= 1.13.0, < 1.14.0`

## Remediation

Upgrade to a patched release:

- `justhtml 1.14.0`