An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3.
Priority LOW 34CWE-79Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Required
- Attack complexity
- Low
- Impact type
- Cross-site scripting
- Confidentiality · integrity · availability
- Low · Low · None
- Fix status
- Partial Red Hat: 5 fixed, 6 without a fix
Attack profile from CVSS 3.1 · CISA ADP. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-79.
Summary
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.
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.04035 90th percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Partial lxml (PyPI): 4.6.3Red Hat: 5 advisoriesRed Hat: 5 fixed, 6 without a fix
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| — | — | all versions | — | No fix listed | CNA |
| — | lxml (PyPI) | before 4.6.3 | 4.6.3 | Patch available | GHSA-jq4v-f5q6-mjqq |
- — Vendor— Affected all versions Fixed in — No fix listed CNA
- lxml (PyPI) Vendor— Affected before 4.6.3 Fixed in 4.6.3 Patch available GHSA-jq4v-f5q6-mjqq
CPE configurations NVD · 7
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:lxml:lxml:*:*:*:*:*:*:*:* | before 4.6.3 | Vulnerable | lxml |
| cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:* | 9.0 | Vulnerable | Debian |
| cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* | 10.0 | Vulnerable | Debian |
| cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:* | 33 | Vulnerable | Fedoraproject |
| cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* | 34 | Vulnerable | Fedoraproject |
| cpe:2.3:a:netapp:snapcenter:-:*:*:*:*:*:*:* | not specified by NVD | Vulnerable | NetApp |
| cpe:2.3:a:oracle:zfs_storage_appliance_kit:8.8:*:*:*:*:*:*:* | 8.8 | Vulnerable | Oracle |
- cpe:2.3:a:lxml:lxml:*:*:*:*:*:*:*:* Versions before 4.6.3 Vulnerable ·lxml
- cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:* Versions 9.0 Vulnerable ·Debian
- cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:* Versions 10.0 Vulnerable ·Debian
- cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:* Versions 33 Vulnerable ·Fedoraproject
- cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:* Versions 34 Vulnerable ·Fedoraproject
- cpe:2.3:a:netapp:snapcenter:-:*:*:*:*:*:*:* Versions not specified by NVD Vulnerable ·NetApp
- cpe:2.3:a:oracle:zfs_storage_appliance_kit:8.8:*:*:*:*:*:*:* Versions 8.8 Vulnerable ·Oracle
Remediation
- Fix · GHSA-jq4v-f5q6-mjqq lxml (PyPI) Fixed in 4.6.3 Advisory →
Red Hat 5 advisories
- RHSA-2021:4160 Released 2021-11-09 · Red Hat rating Moderatepython39:3.9 and python39-devel:3.9 security update
- Red Hat CodeReady Linux Builder (v. 8) Cython-0.29.21-5.module+el8.4.0+9822+20bf1249.src::python39-devel:3.9, pybind11-2.6.1-2.module+el8.4.0+9822+20bf1249.src::python39-devel:3.9, pytest-6.0.2-2.module+el8.4.0+9822+20bf1249.src::python39-devel:3.9 +33
- Red Hat Enterprise Linux AppStream (v. 8) PyYAML-5.4.1-1.module+el8.5.0+10613+59a13ec4.src::python39:3.9, mod_wsgi-4.7.1-4.module+el8.4.0+9822+20bf1249.src::python39:3.9, numpy-1.19.4-3.module+el8.5.0+12204+54860423.src::python39:3.9 +47
- RHSA-2021:4158 Released 2021-11-09 · Red Hat rating Moderatepython-lxml security update
- Red Hat Enterprise Linux AppStream (v. 8) python-lxml-4.2.3-3.el8, python3-lxml-4.2.3-3.el8
- RHSA-2021:4162 Released 2021-11-09 · Red Hat rating Moderatepython38:3.8 and python38-devel:3.8 security update
- Red Hat CodeReady Linux Builder (v. 8) pytest-4.6.6-3.module+el8.4.0+8888+89bc7e79.src::python38-devel:3.8, python-atomicwrites-1.3.0-8.module+el8.4.0+8888+89bc7e79.src::python38-devel:3.8, python-attrs-19.3.0-3.module+el8.4.0+8888+89bc7e79.src::python38-devel:3.8 +15
- Red Hat Enterprise Linux AppStream (v. 8) Cython-0.29.14-4.module+el8.4.0+8888+89bc7e79.src::python38:3.8, PyYAML-5.4.1-1.module+el8.5.0+10721+14d8e0d5.src::python38:3.8, babel-2.7.0-11.module+el8.5.0+11015+9c1c7c42.src::python38:3.8 +47
Show 2 more Red Hat advisories
- RHSA-2021:4151 Released 2021-11-09 · Red Hat rating Moderatepython27:2.7 security update
- Red Hat Enterprise Linux AppStream (v. 8) Cython-0.28.1-7.module+el8.1.0+3111+de3f2d8e.src::python27:2.7, PyYAML-3.12-16.module+el8.1.0+3111+de3f2d8e.src::python27:2.7, babel-2.5.1-10.module+el8.5.0+11014+88fc0d0b.noarch::python27:2.7 +47
- RHSA-2021:3254 Released 2021-08-24 · Red Hat rating Moderaterh-python38 security, bug fix, and enhancement update
- Red Hat Software Collections for Red Hat Enterprise Linux Server (v. 7) rh-python38-babel-2.7.0-12.el7, rh-python38-python-3.8.11-2.el7, rh-python38-python-babel-2.7.0-12.el7 +14
- Red Hat Software Collections for Red Hat Enterprise Linux Server EUS (v. 7.7) rh-python38-babel-2.7.0-12.el7, rh-python38-python-3.8.11-2.el7, rh-python38-python-babel-2.7.0-12.el7 +14
- Red Hat Software Collections for Red Hat Enterprise Linux Workstation (v. 7) rh-python38-babel-2.7.0-12.el7, rh-python38-python-3.8.11-2.el7, rh-python38-python-babel-2.7.0-12.el7 +14
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Ansible Automation Platform 1.2 | Affected | No fix yet |
| Red Hat Ansible Tower 3 | Affected | No fix planned: Out of support scope |
| Red Hat Enterprise Linux 6 | Affected | No fix planned: Out of support scope |
| Red Hat Enterprise Linux 7 | Affected | No fix planned: Out of support scope |
| Red Hat OpenStack Platform 10 (Newton) | Affected | No fix planned: Out of support scope |
| Red Hat OpenStack Platform 13 (Queens) | Affected | No fix planned: Will not fix |
| Red Hat CodeReady Linux Builder (v. 8) | Fixed | RHSA-2021:4160, RHSA-2021:4162 |
| Red Hat Enterprise Linux AppStream (v. 8) | Fixed | RHSA-2021:4151, RHSA-2021:4158, RHSA-2021:4160, RHSA-2021:4162 |
| Red Hat Software Collections for Red Hat Enterprise Linux Server (v. 7) | Fixed | RHSA-2021:3254 |
| Red Hat Software Collections for Red Hat Enterprise Linux Server EUS (v. 7.7) | Fixed | RHSA-2021:3254 |
| Red Hat Software Collections for Red Hat Enterprise Linux Workstation (v. 7) | Fixed | RHSA-2021:3254 |
Not affected: 1 product
- Red Hat Enterprise Linux 9
Timeline
- 2026-10-02 Threatist record created from CVE-2021-28957
- 2026-10-03 Alias added: GHSA-jq4v-f5q6-mjqq
References
Patch 3
Advisory 8
Other 8
- https://github.com/lxml/lxml/pull/316
- https://github.com/lxml/lxml/commit/2d01a1ba8984e0483ce6619b972832377f208a0d
- https://security.netapp.com/advisory/ntap-20210521-0004
- https://pypi.org/project/lxml
- https://lists.fedoraproject.org/archives/list/[email protected]/message/XXN3QPWCTQVOGW4BMWV3AUUZZ4NRZNSQ
- https://lists.fedoraproject.org/archives/list/[email protected]/message/3C2R44VDUY7FJVMAVRZ2WY7XYL4SVN45
- https://github.com/pypa/advisory-database/tree/main/vulns/lxml/PYSEC-2021-19.yaml
- https://github.com/lxml/lxml
Sources
- CVE-2021-28957 · CVE Program
- CVE-2021-28957 · NVD
- FIRST EPSS
- GHSA-jq4v-f5q6-mjqq · GitHub Advisory Database · CC BY 4.0
- CVE-2021-28957 · Red Hat security data · CC BY 4.0
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