CVE-2026-6827

Summary

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.

Affected Software

Weaknesses

• CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

References

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

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