CVE-2026-6827: XSS (CVSS 6.1)

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.

Is your site exposed to CVE-2026-6827?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

6.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS — Exploit Prediction

0.0019

Probability of exploitation

0.09%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-79 Cross-site Scripting (XSS)

References

Other References

https://github.com/EmilStenstrom/justhtml/security/advisories/GHSA-vrx2-77f2-ww34 https://www.vulncheck.com/advisories/justhtml-before-multiple-cross-site-scripting-vulnerabilities

Frequently Asked Questions

What is CVE-2026-6827? +

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. It has a CVSS v3.1 base score of 6.1 (MEDIUM).

How severe is CVE-2026-6827? +

CVE-2026-6827 has a CVSS v3.1 score of 6.1 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0019, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-6827? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-40282

WeGIA is a web manager for charitable institutions. In versions prior to 3.6.10, a Stored Cross-Site Scripting (XSS) vulnerability allows …

CVE-2026-3317

Reflected Cross-Site Scripting (XSS) vulnerability in Navigate Content Management System. The vulnerability is present in the '/blog' endpoint because user …

CVE-2025-10354

Cross-Site Scripting (XSS) vulnerability reflected in Semantic MediaWiki. This vulnerability allows an attacker to execute JavaScript code in the victim's …

CVE-2026-41456

Bludit CMS prior to commit 6732dde contains a reflected cross-site scripting vulnerability in the search plugin that allows unauthenticated attackers …

CVE-2026-40872

mailcow: dockerized is an open source groupware/email suite based on docker. In versions prior to 2026-03b, the admin dashboard's Autodiscover …

CVE-2026-40878

mailcow: dockerized is an open source groupware/email suite based on docker. In versions prior to 2026-03b, the mailcow web interface …

CVE-2026-6827