CVE-2021-43538Network attack vector

UI spoofing via notification race

Published Dec 8, 2021 · Updated Aug 4, 2024

UI spoofing in Mozilla Firefox before 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allows remote attackers to hide security notices. A race in notification code lets a page requesting fullscreen and pointer lock suppress the indicator that those privileges were granted. Successful spoofing requires a user to visit the crafted page and interact while the page wins the timing race.

CVSS severity4.3
Medium
EPSS probability1.16%
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 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allows remote attackers to hide security notices. A race in notification code lets a page requesting fullscreen and pointer lock suppress the indicator that those privileges were granted. Successful spoofing requires a user to visit the crafted page and interact while the page wins the timing race.

The record

CVE
CVE-2021-43538
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-362, CAPEC-26
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 8, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 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 <95

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <95
Mozilla · Thunderbirdversion=unspecified <91.4.0
Mozilla · Firefox ESRversion=unspecified <91.4.0

Published CVSS scores

4.3National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
7.5SUSECVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
4.3National Vulnerability DatabaseAV: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-362Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
CAPEC-26Leveraging Race Conditions

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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