The justhtml Python library contains multiple HTML sanitization bypass issues that let active content such as script or style survive sanitization, opening the door to cross-site scripting. The disclosure carries a CVSS 3.1 base score of 9.8 (CRITICAL), though that score should be read with care; see below.
What Is It
CVE-2026-7808 is an input validation flaw (CWE-20) in justhtml, a Python HTML parsing and sanitization package. Per the NVD record, versions before 1.16.0 contain several distinct bypass paths:
- Policy object reuse/mutation: mutating or reusing sanitization policy objects, including exported defaults, could weaken later sanitization.
- Mixed-case tags in programmatic DOM input:
sanitize()/sanitize_dom()could miss tag names such asScRiPtorStYlE. - Crafted doctype names: programmatically supplied doctype names could serialize into active markup.
- Foreign-content handling: custom policies preserving SVG or MathML could permit animation elements, presentation attributes with external
url(...)references, or DOM trees mislabeled asnamespace="html"to bypass foreign-content checks.
Why It Matters
Sanitizers are typically a last line of defense against XSS, and a bypass in one can silently undermine a security assumption that downstream applications built on top of it may be relying on.
The disclosure assigns CVSS 3.1 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) and CVSS 4.0 9.3; network-reachable, no privileges, no user interaction required. The exploitability metrics are a reasonable fit, but the impact half of that vector does not match the behavior described in the advisory. A sanitizer bypass is a defense-in-depth failure: it yields injected markup executing in the browser context of whatever application consumes the output, which is normally modeled as a scope change (S:C) with partial impact, not as S:U with total confidentiality, integrity, and availability loss of the library host. In particular, nothing in the source material supports an availability impact at all, and the realized severity depends entirely on what the downstream application does with the sanitized output. Treat 9.8 as an upper bound supplied by the disclosure rather than a measured severity for your deployment, and prioritize based on the exposure conditions in the next section.
There is no CISA KEV entry for this CVE in the supplied source material, so no active exploitation is confirmed in that material and no federal remediation deadline applies. NVD status at time of writing is Received, meaning analysis is not yet complete and the published metrics have not been through NVD review.
What's Vulnerable
- Vendor: EmilStenstrom
- Product:
justhtml(PyPI,pkg:pypi/justhtml) - Affected: all versions before 1.16.0
- Not affected: 1.16.0 and later
The advisory notes the issues primarily affect advanced usage rather than the default JustHTML(..., sanitize=True) path for ordinary parsed HTML. Deployments passing programmatic DOM input, reusing or mutating policy objects, or running custom policies that preserve SVG/MathML carry the most exposure, and that gap between default and advanced usage is a further reason the headline score overstates risk for typical integrations.
Patch Status
Fixed in justhtml 1.16.0. Upgrade to 1.16.0 or later. No workaround is documented in the supplied source material.