CVE-2021-28957

Name: CVE-2021-28957
Creator: NVD / NIST
Published: 2021-03-21T05:15:13.367+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 4.00%

Last modified Jun 17, 2026

CVE-2021-28957 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. 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. EPSS estimates a 4.00% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.1/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS Probability

4.00%

89.2th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Lxml	Lxml	< 4.6.3
Debian	Debian Linux	9.0
Debian	Debian Linux	10.0
Fedoraproject	Fedora	33
Fedoraproject	Fedora	34
Netapp	Snapcenter	All versions
Oracle	Zfs Storage Appliance Kit	8.8

References

https://bugs.launchpad.net/lxml/+bug/1888153Exploit, Issue Tracking, Third Party Advisory
https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999Patch, Third Party Advisory
https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270Patch, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00031.htmlMailing List, Third Party Advisory
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://security.gentoo.org/glsa/202208-06Third Party Advisory
https://security.netapp.com/advisory/ntap-20210521-0004/Third Party Advisory
https://www.debian.org/security/2021/dsa-4880Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory
https://bugs.launchpad.net/lxml/+bug/1888153Exploit, Issue Tracking, Third Party Advisory
https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999Patch, Third Party Advisory
https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270Patch, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00031.htmlMailing List, Third Party Advisory
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://security.gentoo.org/glsa/202208-06Third Party Advisory
https://security.netapp.com/advisory/ntap-20210521-0004/Third Party Advisory
https://www.debian.org/security/2021/dsa-4880Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory

Timeline

Published: Mar 21, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-28957?

How severe is CVE-2021-28957?

CVE-2021-28957 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 4.00% probability of exploitation in the next 30 days.

How do I fix CVE-2021-28957?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-28957?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST