CVE-2021-28957 is a medium-severity (CVSS 6.1) Cross-site Scripting (XSS) vulnerability in lxml. 1 public exploit reference exists, so weaponization risk is real. A fix is available for lxml — see the affected versions and patch details below.
lxml vulnerable to Cross-Site Scripting
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-2021-28957.
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-2021-28957 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 377,166 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
An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3. When disabling the safe_attrs_only and forms arguments, the Cleaner class does not remove the formaction attribute allowing for JS to bypass the sanitizer. A remote attacker could exploit this flaw to run arbitrary JS code on users who interact with incorrectly sanitized HTML. This issue is patched in lxml 4.6.3.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | lxml | all versions | 4.6.3pip install --upgrade 'lxml==4.6.3' |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
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 4.6.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-28957 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-2021-28957 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-28957. 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.
Web applications vulnerable to this flaw, where a XSS attack can be accomplished, are only those that use python-lxml to sanitize HTML input and that allow user data to be placed in the "formaction" attribute of a form button. In Red Hat OpenStack Platform, because the flaw has a lower impact and the package is…
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 8 | python27:2.7-8050020210811095446.3e7ace8b | RHSA-2021:4151 |
| Red Hat Enterprise Linux 8 | python-lxml-0:4.2.3-3.el8 | RHSA-2021:4158 |
| Red Hat Enterprise Linux 8 | python39:3.9-8050020210811100211.d428a79b | RHSA-2021:4160 |
| Red Hat Enterprise Linux 8 | python38:3.8-8050020210811101222.e3d35cca | RHSA-2021:4162 |
| Red Hat Software Collections for Red Hat Enterprise Linux 7 | rh-python38-babel-0:2.7.0-12.el7 | RHSA-2021:3254 |
Frequently Asked Questions
Is CVE-2021-28957 in your dependencies?
O3 Security finds CVE-2021-28957 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.