CVE-2021-4296Network attack vector

Cross-site scripting via unescaped validator messages

Published Dec 29, 2022 · Updated Aug 3, 2024

Cross-site scripting in W3C Unicorn allows remote attackers to execute script in a user's browser through a crafted validator message. ValidatorNuMessage assigns the Validator NU JSON message directly to the rendered title without encoding HTML metacharacters. Exploitation requires a user to view the affected validation result, allowing attacker-controlled script to alter content in that browser.

CVSS severity6.1
Medium
EPSS probability0.52%
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 W3C Unicorn allows remote attackers to execute script in a user's browser through a crafted validator message. ValidatorNuMessage assigns the Validator NU JSON message directly to the rendered title without encoding HTML metacharacters. Exploitation requires a user to view the affected validation result, allowing attacker-controlled script to alter content in that browser.

The record

CVE
CVE-2021-4296
Published
Dec 29, 2022
Updated
Aug 3, 2024
Vendor
World Wide Web Consortium (W3C)
Product
Unicorn
Classifications
CWE-79, T1189
Attack vector
network
Privileges
Unavailable

Timeline

How it unfolded

  1. Dec 29, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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=n/a

What conditions does exploitation require?

Attack vectornetwork
Required privilegesUnavailable in this record.

What is affected?

World Wide Web Consortium (W3C) · Unicornversion=n/a

Published CVSS scores

6.1NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
3.5VulDBCVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:N/I:L/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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