CVE intelligence and bounded remediation

CVE-2021-28957 — An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3

Medium CVSS 6.1

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
Medium
CVSS
6.1 (3.1)
Published
2021-03-21
CISA KEV
Not currently listed
Ecosystem
python/pypi
Weaknesses
CWE-79

Affected products

  • lxml / lxml
  • debian / debian_linux / 9.0
  • debian / debian_linux / 10.0
  • fedoraproject / fedora / 33
  • fedoraproject / fedora / 34
  • netapp / snapcenter

Showing 6 representative product identities from 7 source matches. Confirm exact affected versions with the linked vendor and NVD evidence.

Matched remediation archetype

Cross-site scripting and unsafe browser output

This catalog composition supplies bounded fallback guidance. Explicitly reviewed curated workflows load with the complete record below.

Check exposure

  • Trace reflected, stored, and DOM-derived untrusted values into HTML, attributes, URLs, styles, scripts, and client-side template sinks.
  • Identify affected origins, authenticated user roles, sensitive browser capabilities, and where content is shared across tenants.
  • Review framework escaping, rich-text sanitization, legacy templates, and client-side rendering paths.

Remediate safely

  • Use context-aware framework output encoding and safe DOM APIs; keep untrusted data out of executable contexts.
  • Sanitize intentionally supported markup with a maintained allowlist policy and validate URLs and attributes separately.
  • Update affected rendering components and add tests for every output context using inert sentinel markup.

Authoritative sources

Complete CVE record and remediation plan

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