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CVE-2021-28957 lxml

Affected softwarelxml
Severity6.1 Medium CVSS 3.1 3 scores
WeaknessCross-site scripting (CWE-79)
Published21 Mar 2021
Updated17 Jun 2026

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.

Exploitation status

No public exploit

No exploit has been reported in the sources we check. Published 5 years ago.

  • CISA KEV not listed
  • VulnCheck KEV not listed
  • CISA Vulnrichment exploitation: none
  • Indexed PoC none indexed
  • EPSS 4.0% chance in 30 days

Waratek protection

Outside Waratek scope

A Java vulnerability, but not one Waratek protects. It affects only Java on a user's desktop (applets, Web Start, JavaFX, the installer), sits in a product the Waratek agent can't be installed into, or is in a package we don't expect in business applications. The decision below says which.

  • Protection none
  • Action not pursued none
  • Review automated Upstream data
  • Record active
Decided by The only affected product being a sealed appliance image, where Java runs but no -javaagent can be attached
Finding This CVE names only products the Waratek agent cannot be installed into (netapp/snapcenter). They ship as sealed appliance images or as native code with no application JVM, so there is no start-up line to add -javaagent to. The vulnerability may well be a Java one; it is permanently out of ARMR's reach rather than outside its subject.

CVSS

6.1 MEDIUM v3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N 3 metrics

We show the highest score from the newest CVSS version. Scores from different versions can't be compared, and the first score listed is sometimes a 0.0 placeholder.

VersionScoreBandVectorAssignerType
CVSS 3.1 6.1 MEDIUM CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N NVD Primary published
CVSS 3.1 6.1 MEDIUM CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N CISA-ADP Secondary
CVSS 2.0 4.3 none AV:N/AC:M/Au:N/C:N/I:P/A:N NVD Primary

Affected software and releases

Products and versions

5 products, 7 version entries
SourceVendorProductSchemeAffected versions
nvd debian debian linux generic 9.0 · 10.0
nvd fedoraproject fedora generic 33 · 34
nvd lxml lxml generic < 4.6.3
nvd netapp snapcenter generic any version
nvd oracle zfs storage appliance kit generic 8.8

References

URLTags
https://bugs.launchpad.net/lxml/+bug/1888153 Exploit, Issue Tracking, Third Party Advisory
https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999 Patch, Third Party Advisory
https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270 Patch, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00031.html Mailing 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/202208-06 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 Patch, Third Party Advisory

Timeline

Published2021-03-21By the CVE Program.
NVD record modified2026-06-17NVD's own last-modified date for this record.