CVE-2021-43818 is a high-severity (CVSS 7.1) CWE-74 vulnerability in lxml. A fix is available for lxml — see the affected versions and patch details below.
lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-43818.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
lxmlReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | lxml | all versions | 4.6.5pip install --upgrade 'lxml==4.6.5' |
Affected Products
debian linuxdebianfedorafedoraprojectlxmllxmlhci storage node firmwarenetappsolidfirenetappsolidfire enterprise sdsnetappDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for lxml, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update lxml to 4.6.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-43818 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This flaw is rated as Moderate because code execution is limited to the web browser scope. Following products ship affected component, however the vulnerability is not exposed in the product code. Therefore, we are reducing their impact to moderate. * Red Hat Satellite 6.10 * Red Hat Update Infrastructure for Cloud…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.Source: Red Hat security advisory for CVE-2021-43818 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 8 | python39:3.9-8060020220118220913.d3811607 | RHSA-2022:1763 |
| Red Hat Enterprise Linux 8 | python38:3.8-8060020220120164031.5294be16 | RHSA-2022:1764 |
| Red Hat Enterprise Linux 8 | python27:2.7-8060020220210185952.8cdc2268 | RHSA-2022:1821 |
| Red Hat Enterprise Linux 8 | python-lxml-0:4.2.3-4.el8 | RHSA-2022:1932 |
| Red Hat Satellite 6.11 for RHEL 8 | python-lxml-0:4.7.1-1.el8pc | RHSA-2022:5498 |
| Red Hat Software Collections for Red Hat Enterprise Linux 7 | rh-python38-python-lxml-0:4.4.1-8.el7 | RHSA-2022:1664 |
Frequently Asked Questions
Is CVE-2021-43818 in your dependencies?
Find it across PyPI, including transitive dependencies.