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CVE-2021-28957 Moderate

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.

Exploit probability 4.0%
Published March 21, 2021
Required by Not available
Last source change July 8, 2026

02 / AFFECTED SOFTWARE

Affected packages

Unknown Unknown

83 explicit affected versions

PyPI lxml

95 explicit affected versions

PyPI lxml

146 explicit affected versions

03 / CONNECTIONS

Connected vulnerabilities

04 / EVIDENCE

Source records

Open Source Vulnerabilities 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.

View original source
Open Source Vulnerabilities GHSA-jq4v-f5q6-mjqq

An XSS vulnerability was discovered in the python `lxml` 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.

View original source
Open Source Vulnerabilities PYSEC-2021-19

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.

View original source

05 / REFERENCES

Further evidence