lxml_html_clean.Cleaner does not strip javascript: URLs from namespaced URL attributes

Exploitation

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

CVSS

8.2 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:L/A:N

EPSS

• CVE-2026-49825: 0.30% estimated exploitation activity in the next 30 days; percentile 22.46 (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.

lxmlhtmlclean.Cleaner does not strip javascript: URLs from namespaced URL attributes (xlink:href) Reporter: Guillem Lefait <guillem@datamq.com · Date: 2026-05-10 Affected: lxml ≤ 6.1.0 and lxmlhtmlclean ≤ 0.4.4 (latest stable) Confirmed against: lxml 6.1.0 + lxmlhtmlclean 0.4.4 on Python 3.13.5, 3.14.4, and 3.15.0a8 (libxml2 2.14.6 / 2.9.14 — bug is in pure-Python sanitizer logic, independent of the libxml2 backend) Root-cause class: same as CVE-2021-28957 (formaction missing from linkattrs) Summary Cleaner filters URL schemes (javascript:, vbscript:, …) by walking links via rewritelinks(), which delegates to iterlinks(), which only yields attributes named in lxml.html.defs.linkattrs. That allow-list contains no prefixed names (xlink:href) and no srcset. As a result, when Cleaner is configured with safeattrsonly=False — a documented option for callers that want lenient attribute handling but still expect URL-scheme scrubbing — <a xlink:href="javascript:…" survives sanitization…

Published by the source

8 Jul 2026, 20:23 UTC

Last modified by the source

13 Jul 2026, 16:42 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4jhm-jv67-739f

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4jhm-jv67-739f

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# Patch Next advisory brief: CVE-2026-49825

Advisory: `lxml_html_clean.Cleaner` does not strip `javascript:` URLs from namespaced URL attributes
IDs: CVE-2026-49825, GHSA-4jhm-jv67-739f
Record: https://patchnext.com/advisories/cve-2026-49825 (revision 2, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lxml-html-clean 0.4.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI lxml-html-clean: affected < 0.4.5; fixed in 0.4.5

## Suggested inspection (read-only)
- `python -m pip show lxml-html-clean`: 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 lxml-html-clean.
- 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-4jhm-jv67-739f (OSV record GHSA-4jhm-jv67-739f lists affected ranges: PyPI lxml-html-clean >=0 <0.4.5 (fixed))
- Verified: https://github.com/advisories/GHSA-4jhm-jv67-739f (GitHub-reviewed advisory GHSA-4jhm-jv67-739f lists affected ranges: PyPI lxml-html-clean >=0 <0.4.5)

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