CVE-2021-4128Public exploitNetwork attack vector

Browser crash via unlocked fullscreen layer iteration

Published Dec 22, 2022 · Updated Apr 16, 2025

Use-after-free in Firefox before 95 on macOS allows remote attackers to crash the browser through a crafted fullscreen transition. NativeLayerRootCA::SetWindowIsFullscreen iterates mSublayers while another thread can mutate the collection, leaving a graphics object unprotected. A user must interact with attacker-controlled content that drives rapid fullscreen entry and exit; the bounded reported impact is memory corruption and a browser crash.

CVSS severity6.5
Medium
EPSS probability0.55%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Firefox before 95 on macOS allows remote attackers to crash the browser through a crafted fullscreen transition. NativeLayerRootCA::SetWindowIsFullscreen iterates mSublayers while another thread can mutate the collection, leaving a graphics object unprotected. A user must interact with attacker-controlled content that drives rapid fullscreen entry and exit; the bounded reported impact is memory corruption and a browser crash.

The record

CVE
CVE-2021-4128
Published
Dec 22, 2022
Updated
Apr 16, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-416, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 22, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 16, 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?

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

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

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.

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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-416Use After Free
T1203Exploitation for Client Execution

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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