Is CVE-2021-4129 actively exploited?

No active exploitation of CVE-2021-4129 has been confirmed so far. Continue monitoring, as exploit status can change.

Can CVE-2021-4129 be exploited remotely?

Yes. CVE-2021-4129 is exploitable remotely over the network (CVSS attack vector: Network).

What is the CVSS score of CVE-2021-4129?

CVE-2021-4129 has a CVSS score of 9.8 (CRITICAL).

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CRITICAL

CVE-2021-4129

CWE-787Published: December 22, 2022· Updated: Jun 17, 2026

9.8

CVSS v3.1

Official Description

Mozilla developers and community members Julian Hector, Randell Jesup, Gabriele Svelto, Tyson Smith, Christian Holler, and Masayuki Nakano reported memory safety bugs present in Firefox 94. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Firefox < 95, Firefox ESR < 91.4.0, and Thunderbird < 91.4.0.

NVD Source

Risk Analysis

Memory safety bugs were discovered that could lead to memory corruption and potential arbitrary code execution. The critical severity rating reflects the high risk of system compromise.

No public exploit is known, and it is not in the KEV catalog. The flaw is remotely exploitable.

Recommended Action

Update to Firefox 95, Firefox ESR 91.4.0, or Thunderbird 91.4.0 to resolve the memory safety issues.

Generated by the CTIWATCH analysis pipeline from this CVE's metadata (CVSS, EPSS, KEV status, exploit intelligence). Verify against vendor advisories before acting.

Technical Analysis

CVE-2021-4129 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in complete confidentiality breach (data exposure), full integrity compromise (data manipulation), availability disruption (denial of service), with a CVSS base score of 9.8.

From a weakness classification perspective (CWE-787): Out-of-bounds write vulnerabilities can lead to data corruption, crashes, or arbitrary code execution.