CVE-2026-5751
CVE-2026-5751
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.
Vulnerability class: XSS (Cross-Site Scripting)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (16.6th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 17th percentile — 16.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001
Weakness classification (CWE)
References
- github.com/EmilStenstrom/justhtml/security/advisories/GHSA-r758-8hxw-4845 (vendor-advisory)
- www.vulncheck.com/advisories/justhtml-before-mutation-xss-via-custom-sanitizati… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-5751?
- CVE-2026-5751 is a medium-severity vulnerability, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2026-08-23.
- How severe is CVE-2026-5751?
- Medium severity. CVSS v3 base score is 6.1 out of 10.