# CVE-2021-28957: lxml vulnerable to Cross-Site Scripting

Source: https://junglewise.ai/threats/cve-2021-28957-lxml-vulnerable-to-cross-site-scripting  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2021-03-21  
License: CC BY 4.0 with a link back

CVE-2021-28957 · Severity: low · CVSS 3.1 · Published 2021-03-21

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.

Technologies: [lxml (PyPI)](https://junglewise.ai/threats/technologies/lxml). Vendors: [PyPI](https://junglewise.ai/threats/vendors/pypi).

## Quick answers

**Is CVE-2021-28957 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2021-28957 being exploited in the wild.

**How severe is CVE-2021-28957?** Low severity, with a CVSS score of 3.1, published 21 March 2021.

**Which products and versions are affected?** lxml (PyPI) ECOSYSTEM:introduced=0,fixed=4.6.3, GIT:introduced=0,fixed=a5f9cb52079dc57477c460dbe6ba0f775e14a999|ECOSYSTEM:introduced=0,fixed=4.6.3.

**Is there a fix for CVE-2021-28957?** Yes. lxml (PyPI): upgrade to 4.6.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**Am I affected by CVE-2021-28957?** Check whether you run lxml (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- lxml (PyPI): upgrade to 4.6.3 or later

  ```sh
  pip install "lxml>=4.6.3"
  ```


## Affected products

- lxml (PyPI) ECOSYSTEM:introduced=0,fixed=4.6.3, GIT:introduced=0,fixed=a5f9cb52079dc57477c460dbe6ba0f775e14a999|ECOSYSTEM:introduced=0,fixed=4.6.3

## Identifiers

- CVE-2021-28957
- GHSA-jq4v-f5q6-mjqq
- PYSEC-2021-19

## Related threats

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- [lxml NULL Pointer Dereference allows attackers to cause a denial of service](https://junglewise.ai/threats/cve-2022-2309-lxml-null-pointer-dereference-allows-attackers-to-cause-a-denial)
- [lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through](https://junglewise.ai/threats/cve-2021-43818-lxml-s-html-cleaner-allows-crafted-and-svg-embedded-scripts-to)
- [lxml vulnerable to Cross-site Scripting](https://junglewise.ai/threats/cve-2020-27783-lxml-vulnerable-to-cross-site-scripting)
- [Improper Neutralization of Input During Web Page Generation in LXML](https://junglewise.ai/threats/cve-2018-19787-improper-neutralization-of-input-during-web-page-generation-in)