lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through

Exploitation

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

CVSS 4.0

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

EPSS

• CVE-2021-43818: 2.66% estimated exploitation activity in the next 30 days; percentile 85.73 (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.

Impact The HTML Cleaner in lxml.html lets certain crafted script content pass through, as well as script content in SVG files embedded using data URIs. Users that employ the HTML cleaner in a security relevant context should upgrade to lxml 4.6.5. Patches The issue has been resolved in lxml 4.6.5. Workarounds None. References The issues are tracked under the report IDs GHSL-2021-1037 and GHSL-2021-1038.

Published by the source

13 Dec 2021, 18:14 UTC

Last modified by the source

8 Jul 2026, 06:28 UTC

Source last checked

26 Oct 2026, 13:21 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-55x5-fj6c-h6m8

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-55x5-fj6c-h6m8

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# Patch Next advisory brief: CVE-2021-43818

Advisory: lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through
IDs: CVE-2021-43818, GHSA-55x5-fj6c-h6m8
Record: https://patchnext.com/advisories/cve-2021-43818 (revision 3, source checked 26 Oct 2026, 13:21 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: lxml 4.6.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI lxml: affected < 4.6.5; fixed in 4.6.5

## Suggested inspection (read-only)
- `python -m pip show lxml`: 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.
- 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-55x5-fj6c-h6m8 (OSV record GHSA-55x5-fj6c-h6m8 lists affected ranges: PyPI lxml >=0 <4.6.5 (fixed))
- Verified: https://github.com/advisories/GHSA-55x5-fj6c-h6m8 (GitHub-reviewed advisory GHSA-55x5-fj6c-h6m8 lists affected ranges: PyPI lxml >=0 <4.6.5)

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