CVE-2021-4129
CVSS v3
9.8
CRITICAL
EPSS Score
1.0 %
exploit probability, as of 2026-09-25
CISA KEV
No
known exploited
Exploitation
—
SSVC status
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.
Technical details
- Published
- 2022-12-22
Frequently asked questions
What is CVE-2021-4129?
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.
Is CVE-2021-4129 actively exploited?
Active exploitation of CVE-2021-4129 has not been confirmed. Its EPSS score was 1.0% on 2026-09-25, the estimated probability of exploitation in the next 30 days.
What is the CVSS score for CVE-2021-4129?
CVE-2021-4129 has a CVSS v3 base score of 9.8 (CRITICAL severity).
Is CVE-2021-4129 affecting your environment?
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