justhtml has sanitization bypass in custom policies and programmatic DOM

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

6.0 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:L/SA:N · score source

EPSS

• CVE-2026-6827: 0.26% estimated exploitation activity in the next 30 days; percentile 17.96 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary justhtml 1.17.0 fixes multiple security issues in sanitization, serialization, and programmatic DOM handling. Most of these issues affected advanced or custom configurations rather than the default safe path. Affected versions - justhtml <= 1.16.0 Fixed version - justhtml 1.17.0 released on April 19, 2026 Impact Custom SVG / MathML sanitization policies Custom policies that preserved foreign namespaces could allow dangerous content to survive sanitization, including: - active HTML integration points such as SVG <foreignObject, MathML <annotation-xml encoding="text/html", SVG <title / <desc, and MathML text integration points - mutation-XSS parser-differential payloads that looked inert in memory but became active HTML after reparse - SVG filter="url(...)" attributes that could trigger external fetches These issues affected: - JustHTML(..., sanitize=True) with custom foreign-namespace policies - sanitize() / sanitizedom() - low-level terminal Sanitize(...) transform execution…

Published by the source

22 Apr 2026, 21:25 UTC

Last modified by the source

24 Aug 2026, 03:56 UTC

Source last checked

26 Oct 2026, 13:40 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-vrx2-77f2-ww34

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-vrx2-77f2-ww34

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# Patch Next advisory brief: CVE-2026-6827

Advisory: justhtml has sanitization bypass in custom policies and programmatic DOM
IDs: CVE-2026-6827, GHSA-vrx2-77f2-ww34
Record: https://patchnext.com/advisories/cve-2026-6827 (revision 3, source checked 26 Oct 2026, 13:40 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.0)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: justhtml 1.17.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI justhtml: affected < 1.17.0; fixed in 1.17.0

## Suggested inspection (read-only)
- `python -m pip show justhtml`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on justhtml.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-vrx2-77f2-ww34 (OSV record GHSA-vrx2-77f2-ww34 lists affected ranges: PyPI justhtml >=0 <1.17.0 (fixed))
- Verified: https://github.com/advisories/GHSA-vrx2-77f2-ww34 (GitHub-reviewed advisory GHSA-vrx2-77f2-ww34 lists affected ranges: PyPI justhtml >=0 <1.17.0)

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