CVE-2026-6827 justhtml before 1.17.0 Multiple Cross-Site Scripting Vulnerabilities
Attackers can execute arbitrary scripts via XSS in justhtml when using custom policies that preserve foreign namespaces or specific DOM structures.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-6827?
CVE-2026-6827 is a medium-severity cross-site scripting vulnerability in EmilStenstrom justhtml, published on 23 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
justhtml before 1.17.0 contains multiple security issues in sanitization, serialization, and programmatic DOM handling. When custom policies preserve foreign namespaces (SVG/MathML), dangerous content such as HTML integration points (SVG <foreignObject>, MathML <annotation-xml encoding="text/html">) and mutation-XSS parser-differential payloads could survive sanitization and become active HTML after reparse; SVG filter="url(...)" and preserved <style> could leave resource-loading CSS (@import, background-image:url()). Programmatic script/style/Comment nodes could serialize into active markup. Additional hardening fixes address sanitize-pipeline cache mutation and DOM parent/child cycles that could cause infinite loops. Most issues affect advanced or custom configurations rather than the default JustHTML(..., sanitize=True) safe path.
Weakness: CWE-79
Which versions are affected by CVE-2026-6827?
2 open source packages ship an affected version, including justhtml.
| Package | Affected | Fixed in |
|---|---|---|
justhtml PyPI | All versions | 1.17.0 |
justhtml PyPI | All versions | 1.17.0 |
How do I fix CVE-2026-6827?
Upgrade justhtml to 1.17.0 or later.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-6827 being exploited?
There is no known exploitation of CVE-2026-6827 in the wild and no public exploit code.
Sources
Also known as GHSA-f8m6-5gph-fw7f, GHSA-vrx2-77f2-ww34.