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Vulnerability record · CVE-2021-28957 · published 21 March 2021

CVE-2021-28957: Lxml cross-site scripting vulnerability

LLxml · Lxml

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.

6.1 CVSS 3.1 Medium EPSS 4.0% · top 9.7% CWE-79 · Cross-site scripting
6.1CVSS 3.1 base score, v2 4.3
4.0%EPSS exploitation probability, 30 days
NoNot in CISA KEV
5Affected product versions listed by NVD
20References, 2 tagged exploit
17 Jun 2026Last modified by NVD

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.

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

Affected products

5 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bugs.launchpad.net/lxml/+bug/1888153 ExploitIssue TrackingThird Party Advisory
https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999 PatchThird Party Advisory
https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270 PatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00031.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3C2R44VDUY7FJVMAVRZ2WY7
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XXN3QPWCTQVOGW4BMWV3AUU
https://security.gentoo.org/glsa/202208-06 Third Party Advisory
https://security.netapp.com/advisory/ntap-20210521-0004/ Third Party Advisory
https://www.debian.org/security/2021/dsa-4880 Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory
https://bugs.launchpad.net/lxml/+bug/1888153 ExploitIssue TrackingThird Party Advisory
https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999 PatchThird Party Advisory
https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270 PatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00031.html Mailing ListThird Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/3C2R44VDUY7FJVMAVRZ2WY7
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/XXN3QPWCTQVOGW4BMWV3AUU
https://security.gentoo.org/glsa/202208-06 Third Party Advisory
https://security.netapp.com/advisory/ntap-20210521-0004/ Third Party Advisory
https://www.debian.org/security/2021/dsa-4880 Third Party Advisory
https://www.oracle.com/security-alerts/cpuoct2021.html PatchThird Party Advisory

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Source: NIST National Vulnerability Database (record CVE-2021-28957), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.