Summary
Workarounds
None.
References
The issues are tracked under the report IDs GHSL-2021-1037 and GHSL-2021-1038.
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.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2021-43818 has a CVSS score of 8.2 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.6.5); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
The issue has been resolved in lxml 4.6.5.
Frequently Asked Questions
- What is CVE-2021-43818? CVE-2021-43818 is a medium-severity cross-site scripting (XSS) vulnerability in lxml (pip), affecting versions < 4.6.5. It is fixed in 4.6.5. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2021-43818? CVE-2021-43818 has a CVSS score of 8.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of lxml are affected by CVE-2021-43818? lxml (pip) versions < 4.6.5 is affected.
- Is there a fix for CVE-2021-43818? Yes. CVE-2021-43818 is fixed in 4.6.5. Upgrade to this version or later.
- Is CVE-2021-43818 exploitable, and should I be worried? Whether CVE-2021-43818 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-43818 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-43818? Upgrade
lxmlto 4.6.5 or later.