CVE-2021-43818Network attack vector

Cross-site scripting via HTML Cleaner sanitization bypass

Published Dec 13, 2021 · Updated Dec 18, 2025

lxml before 4.6.5 allows remote attackers to deliver crafted HTML that survives the HTML Cleaner and reaches downstream browsers for cross-site scripting. The cleaner's regular-expression handling of inline style attributes can be bypassed, and data:image SVG URLs are accepted despite embedded script elements. Exploitation requires an application to sanitize attacker-controlled HTML with lxml and a user or browser to render the resulting content; confidentiality and integrity impact depend on that downstream context.

CVSS severity8.2
High
EPSS probability2.48%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

lxml before 4.6.5 allows remote attackers to deliver crafted HTML that survives the HTML Cleaner and reaches downstream browsers for cross-site scripting. The cleaner's regular-expression handling of inline style attributes can be bypassed, and data:image SVG URLs are accepted despite embedded script elements. Exploitation requires an application to sanitize attacker-controlled HTML with lxml and a user or browser to render the resulting content; confidentiality and integrity impact depend on that downstream context.

The record

CVE
CVE-2021-43818
Published
Dec 13, 2021
Updated
Dec 18, 2025
Vendor
lxml
Product
lxml
Classifications
CWE-79, CWE-74, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 18, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.6.5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

lxml · lxmlversion=< 4.6.5

Published CVSS scores

7.1NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L
6.8NVDAV:N/AC:M/Au:N/C:P/I:P/A:P
8.2GitHub CNACVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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