CVE-2021-29961Public exploitNetwork attack vector

UI spoofing via missing picture-cache iframe clipping

Published Jun 24, 2021 · Updated Aug 3, 2024

UI spoofing in Mozilla Firefox before 89 allows remote attackers to paint attacker-controlled content over browser chrome via a crafted page. WebRender maps an extreme CSS transform into primitive space with insufficient floating-point precision, while picture-cache tiles lack the iframe clip that should confine page content. A user must open the page with WebRender enabled; the page can cover the address bar or toolbar but does not gain code execution or data access.

CVSS severity4.3
Medium
EPSS probability0.76%
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 89 allows remote attackers to paint attacker-controlled content over browser chrome via a crafted page. WebRender maps an extreme CSS transform into primitive space with insufficient floating-point precision, while picture-cache tiles lack the iframe clip that should confine page content. A user must open the page with WebRender enabled; the page can cover the address bar or toolbar but does not gain code execution or data access.

The record

CVE
CVE-2021-29961
Published
Jun 24, 2021
Updated
Aug 3, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-863
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 <89

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <89

Published CVSS scores

4.3NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
4.3NVDCVSS:3.1/AV:N/AC:L/PR:N/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 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-863Incorrect Authorization

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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