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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-43818 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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' |
Detection & 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
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-43818 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-43818. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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…
| 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?
O3 Security finds CVE-2021-43818 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.