CVE-2026-5751

Summary

justhtml versions 1.13.0 and earlier contain a parser-differential / mutation cross-site scripting (mXSS) vulnerability when using a custom SanitizationPolicy that preserves foreign namespaces (e.g., drop_foreign_namespaces=False with allowlisted SVG/MathML elements or raw-text containers such as <style>). Specially crafted input can sanitize into markup that appears safe but becomes unsafe when re-parsed by a browser or another HTML parser, allowing markup injection. The default safe configuration (sanitize=True) is not affected. Fixed in 1.14.0.

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-r758-8hxw-4845
• https://www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitization-policies

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