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MEDIUM

CVE-2021-28957

CVSS v3

6.1

MEDIUM

EPSS Score

4.0 %

exploit probability, as of 2026-10-19

CISA KEV

No

known exploited

Exploitation

—

SSVC status

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Affected and fixed versions

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • lxmlPyPIall versions · fixed in 4.6.3
  • babelAlmaLinux:8all versions · fixed in 2.5.1-10.module_el8.6.0+2781+fed64c13and 1 other package
  • python-nose-docsAlmaLinux:8all versions · fixed in 1.3.7-31.module_el8.5.0+2569+5c5719bc
  • python-sqlalchemy-docAlmaLinux:8all versions · fixed in 1.3.2-2.module_el8.5.0+2569+5c5719bc
  • python2-attrsAlmaLinux:8all versions · fixed in 17.4.0-10.module_el8.6.0+2781+fed64c13
  • python2-backportsAlmaLinux:8all versions · fixed in 1.0-16.module_el8.6.0+2781+fed64c13
  • python2-backports-ssl_match_hostnameAlmaLinux:8all versions · fixed in 3.5.0.1-12.module_el8.6.0+2781+fed64c13
  • python2-chardetAlmaLinux:8all versions · fixed in 3.0.4-10.module_el8.6.0+2781+fed64c13

99 more package ranges on OSV.dev

Vendor advisories

  • CVE-2021-28957 Red Hat Product Security871 products fixed · 7 known affected

    python-lxml: Missing input sanitization for formaction HTML5 attributes may lead to XSS

    lxml.src as a component of Red Hat Ansible Automation Platform 1.2 · lxml.src as a component of Red Hat Ansible Tower 3 · python-lxml.src as a component of Red Hat Enterprise Linux 6

  • RHSA-2021:4160 Red Hat Product Security229 products fixed

    Red Hat Security Advisory: python39:3.9 and python39-devel:3.9 security update

  • RHSA-2021:4158 Red Hat Product Security13 products fixed

    Red Hat Security Advisory: python-lxml security update

  • RHSA-2021:4162 Red Hat Product Security239 products fixed

    Red Hat Security Advisory: python38:3.8 and python38-devel:3.8 security update

  • RHSA-2021:4151 Red Hat Product Security264 products fixed

    Red Hat Security Advisory: python27:2.7 security update

  • RHSA-2021:3254 Red Hat Product Security126 products fixed

    Red Hat Security Advisory: rh-python38 security, bug fix, and enhancement update

Full record on 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.

Technical details

Published
2021-03-21

Frequently asked questions

What is CVE-2021-28957?

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.

Is CVE-2021-28957 actively exploited?

Active exploitation of CVE-2021-28957 has not been confirmed. Its EPSS score was 4.0% on 2026-10-19, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-28957?

CVE-2021-28957 has a CVSS v3 base score of 6.1 (MEDIUM severity).

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