Vulnerability-Lookup

RHSA-2021:4158

Vulnerability from csaf_redhat - Published: 2021-11-09 18:14 - Updated: 2026-11-06 17:14

Summary

Red Hat Security Advisory: python-lxml security update

Severity

Moderate

Notes

Topic: An update for python-lxml is now available for Red Hat Enterprise Linux 8. Red Hat Product Security has rated this update as having a security impact of Moderate. A Common Vulnerability Scoring System (CVSS) base score, which gives a detailed severity rating, is available for each vulnerability from the CVE link(s) in the References section.

Details: lxml is an XML processing library providing access to libxml2 and libxslt libraries using the Python ElementTree API. Security Fix(es): * python-lxml: Missing input sanitization for formaction HTML5 attributes may lead to XSS (CVE-2021-28957) For more details about the security issue(s), including the impact, a CVSS score, acknowledgments, and other related information, refer to the CVE page(s) listed in the References section. Additional Changes: For detailed information on changes in this release, see the Red Hat Enterprise Linux 8.5 Release Notes linked from the References section.

Terms of Use: This content is licensed under the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/). If you distribute this content, or a modified version of it, you must provide attribution to Red Hat Inc. and provide a link to the original.

CVE-2021-28957

A flaw was found in python-lxml. The HTML5 formaction attribute is not input sanitized like the HTML action attribute is which can lead to a Cross-Site Scripting attack (XSS) when an application uses python-lxml to sanitize user inputs. The highest threat from this vulnerability is to data confidentiality and integrity as well as system availability.

6.1 (Medium)


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

CWE-79 - Improper Neutralization of Input During Web Page Generation

Affected products

Fixed 13 products

Product	Version	Remediation
Unresolved product id: AppStream-8.5.0.GA:python-lxml-0:4.2.3-3.el8.src	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python-lxml-debugsource-0:4.2.3-3.el8.aarch64	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python-lxml-debugsource-0:4.2.3-3.el8.ppc64le	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python-lxml-debugsource-0:4.2.3-3.el8.s390x	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python-lxml-debugsource-0:4.2.3-3.el8.x86_64	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-0:4.2.3-3.el8.aarch64	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-0:4.2.3-3.el8.ppc64le	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-0:4.2.3-3.el8.s390x	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-0:4.2.3-3.el8.x86_64	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-debuginfo-0:4.2.3-3.el8.aarch64	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-debuginfo-0:4.2.3-3.el8.ppc64le	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-debuginfo-0:4.2.3-3.el8.s390x	—	Vendor Fix fix
Unresolved product id: AppStream-8.5.0.GA:python3-lxml-debuginfo-0:4.2.3-3.el8.x86_64	—	Vendor Fix fix

Threats

Impact Moderate

References

9 references

URL	Category
https://access.redhat.com/errata/RHSA-2021:4158	self
https://access.redhat.com/security/updates/classi…	external
https://access.redhat.com/documentation/en-us/red…	external
https://bugzilla.redhat.com/show_bug.cgi?id=1941534	external
https://security.access.redhat.com/data/csaf/v2/a…	self
https://access.redhat.com/security/cve/CVE-2021-28957	self
https://bugzilla.redhat.com/show_bug.cgi?id=1941534	external
https://www.cve.org/CVERecord?id=CVE-2021-28957	external
https://nvd.nist.gov/vuln/detail/CVE-2021-28957	external

JSON

To clipboard

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          "title": "Vulnerability description"
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        },
        {
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      "title": "python-lxml: Missing input sanitization for formaction HTML5 attributes may lead to XSS"
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}

CVE-2021-28957 (GCVE-0-2021-28957)

Vulnerability from cvelistv5 – Published: 2021-03-21 04:39 – Updated: 2025-12-17 21:31

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.

Severity

6.1 (Medium)


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

SSVC

Exploitation: none Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2025-12-17 21:31 UTC

CWE

n/a
CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Assigner

mitre CNA under the mitre root

CNA scorecard E 29/100 over 2159 records in the last 180 days details

References

10 references

URL	Tags
https://bugs.launchpad.net/lxml/+bug/1888153	x_refsource_MISC
https://github.com/lxml/lxml/pull/316/commits/10e…	x_refsource_MISC
https://lists.debian.org/debian-lts-announce/2021…	mailing-listx_refsource_MLIST
https://www.debian.org/security/2021/dsa-4880	vendor-advisoryx_refsource_DEBIAN
https://lists.fedoraproject.org/archives/list/pac…	vendor-advisoryx_refsource_FEDORA
https://lists.fedoraproject.org/archives/list/pac…	vendor-advisoryx_refsource_FEDORA
https://www.oracle.com/security-alerts/cpuoct2021.html	x_refsource_MISC
https://github.com/lxml/lxml/commit/a5f9cb52079dc…	x_refsource_MISC
https://security.netapp.com/advisory/ntap-2021052…	x_refsource_CONFIRM
https://security.gentoo.org/glsa/202208-06	vendor-advisoryx_refsource_GENTOO

Show details on NVD website

JSON

To clipboard

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Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Vulnerability timeline

Sightings and recorded dates, grouped by day · UTC

Order JSON

How durations are calculated

Following the VTEM framework (Black Hat USA 2015), time to patch starts when the vendor or a coordinator was informed, total time to patch at discovery, and disclosure to patch at public disclosure; vendor response time runs from the report to its acknowledgement. Time to exploit and time unpatched start at the earliest discovery or report, falling back to public disclosure. Unpatched time ends at the first recorded patch, or the calculation time when no patch is recorded. Exploitation to patch shows the interval between those two events; a published proof of concept is a public exploit, not exploitation.

Each result shows its source dates. A negative interval means the end event came first; when date-only inputs leave the order open, the result says so. Date-only inputs are approximate. The earliest evidence wins: KEV report dates and sightings are labelled as such. Records describing another vulnerability are listed but never calculated from, and a PoC alone does not establish exploitation. Missing patch evidence does not prove that no patch exists. Filters below change the event list only.

Publication & reservation Sightings KEV catalogs Other timestamps

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

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