Is CVE-2021-4129 in the CISA KEV catalog?

No. CVE-2021-4129 is not currently on the CISA Known Exploited Vulnerabilities list.

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

CVE-2021-4129 has a CVSS v3 base score of 9.8 (CRITICAL).

What is the EPSS score for CVE-2021-4129?

CVE-2021-4129 has an EPSS score of 1.01% (59th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-4129?

Upgrade firefox on debian unstable to 95.0-1 or later. Fixes are available for 6 affected products.

CVE & CISA-KEV Catalog

CVE-2021-4129

CRITICAL

9.8

CVSS v3

NVD

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.

CWE-787Out-of-bounds Write

How to fix

Remediation Available

firefoxDebian

Fixed in:95.0-1CVE-2021-4129

thunderbirdUbuntu

Fixed in:1:91.5.0+build1-0ubuntu0.18.04.1USN-5248-1

Fixed in:1:91.5.0+build1-0ubuntu0.20.04.1USN-5248-1

DeveloperEditionWindows application

Affected:95.0Fixed in:95.0Mozilla

Mozilla Firefox (en-US)Windows application

Affected:95.0Fixed in:95.0Mozilla

Mozilla Firefox ESR (en-US)Windows application

Affected:91.4.0Fixed in:91.4.0Mozilla

Mozilla Thunderbird (en-US)Windows application

Affected:91.4.0Fixed in:91.4.0Mozilla

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeUnchanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Exploit Intelligence

1.01%probability of exploitation in 30 days

59thpercentile

Moderate risk: more likely to be exploited than 59% of all known CVEs.

References

Exploit1

bugzilla.mozilla.org

Vendor Advisory3

Related Vulnerabilities

Other CWE-787 (Out-of-bounds Write) vulnerabilities, ordered by exploit likelihood. View all

CVE	Severity	CVSS	EPSS	Exploited	Fix
CVE-2025-22457	Critical	9.0	100%	KEV + Ransom	Fix
CVE-2025-0282	Critical	9.0	100%	KEV + Ransom	-
CVE-2015-3113	Critical	9.8	100%	KEV	Fix
CVE-2021-20038	Critical	9.8	100%	KEV + Ransom	-
CVE-2023-4863	High	8.8	100%	KEV	Fix
CVE-2020-16040	Medium	6.5	100%	-	Fix

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-04-16.