CVE-2021-28957Public exploitNetwork attack vector

Cross-site scripting via retained formaction attribute

Published Mar 21, 2021 · Updated Dec 17, 2025

Cross-site scripting in lxml Cleaner before 4.6.3 allows remote attackers to execute JavaScript through sanitized HTML. The cleaner's link-attribute filtering omits HTML5 formaction, so Cleaner retains a crafted JavaScript-bearing attribute when safe_attrs_only and forms are disabled. Exploitation requires an application to use those nondefault options and a user to interact with the incorrectly sanitized output.

CVSS severity6.1
Medium
EPSS probability4.04%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Cross-site scripting in lxml Cleaner before 4.6.3 allows remote attackers to execute JavaScript through sanitized HTML. The cleaner's link-attribute filtering omits HTML5 formaction, so Cleaner retains a crafted JavaScript-bearing attribute when safe_attrs_only and forms are disabled. Exploitation requires an application to use those nondefault options and a user to interact with the incorrectly sanitized output.

The record

CVE
CVE-2021-28957
Published
Mar 21, 2021
Updated
Dec 17, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-79, T1204.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 21, 2021CVE publishedPublication date reported by the CVE source.
  2. Dec 17, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
5.3GitHub Advisory DatabaseCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/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 exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204.001Malicious Link

Public exploit references

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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