CVE-2021-4129Network attack vector

Code execution via undisclosed memory corruption

Published Dec 22, 2022 · Updated Apr 16, 2025

Memory-safety flaws in Mozilla Firefox before 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allow attackers to execute code. Mozilla identifies six underlying bugs and observed memory corruption, but the public advisories do not disclose the faulty operations or affected code loci. Untrusted browser or browser-like content must reach an affected code path; Thunderbird generally blocks this route while reading email because scripting is disabled.

CVSS severity9.8
Critical
EPSS probability1.01%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Memory-safety flaws in Mozilla Firefox before 95, Firefox ESR before 91.4.0, and Thunderbird before 91.4.0 allow attackers to execute code. Mozilla identifies six underlying bugs and observed memory corruption, but the public advisories do not disclose the faulty operations or affected code loci. Untrusted browser or browser-like content must reach an affected code path; Thunderbird generally blocks this route while reading email because scripting is disabled.

The record

CVE
CVE-2021-4129
Published
Dec 22, 2022
Updated
Apr 16, 2025
Vendor
Mozilla
Product
Firefox
Classifications
CWE-787, 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
Mozilla · Thunderbirdversion=unspecified <91.4.0
Mozilla · Firefox ESRversion=unspecified <91.4.0

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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.

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1203Exploitation for Client Execution

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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