CVE-2021-23996Public exploitNetwork attack vector

UI spoofing via unclipped 3D web content

Published Jun 24, 2021 · Updated Aug 3, 2024

UI spoofing in Mozilla Firefox before 88 allows remote attackers to overlay browser chrome with web content through a crafted webpage. WebRender fails to clip transformed 3D CSS content to the webpage viewport when JavaScript constructs the rendered elements, allowing arbitrary page content to cover the address and tab bars. A user must load the crafted page; the demonstrated consequence is deceptive browser-interface content suitable for phishing, not code execution or direct data modification.

CVSS severity6.5
Medium
EPSS probability0.72%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

UI spoofing in Mozilla Firefox before 88 allows remote attackers to overlay browser chrome with web content through a crafted webpage. WebRender fails to clip transformed 3D CSS content to the webpage viewport when JavaScript constructs the rendered elements, allowing arbitrary page content to cover the address and tab bars. A user must load the crafted page; the demonstrated consequence is deceptive browser-interface content suitable for phishing, not code execution or direct data modification.

The record

CVE
CVE-2021-23996
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
Unavailable
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2021CVE 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=unspecified <88

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <88

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/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 exploitPublished

Weakness, pattern, technique

No sourced classifications are available.

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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