CVE-2021-4129: mozilla firefox Out-of-bounds Write
Vulnerability summary
CVE-2021-4129 is a critical (CVSS 9.8) Out-of-bounds Write vulnerability in mozilla firefox. EPSS estimates a 1.02% chance of exploitation in the next 30 days, higher than 62.9% of scored vulnerabilities. 1 public exploit is recorded.
Advisory lineage
Upstream 0 · Downstream 15 · Related 1Timeline
- 22-12-2022PublishedVulnerability first disclosed.
- 16-04-2025Last modifiedVulnerability information updated.
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.
CVSS metrics
4 scores · 4 sources · 1 version- OSV · UbuntuEcosystem advisory · calculated9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- OSV · DebianEcosystem advisory · calculated9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- NVDNIST9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS trend
Probability
1.02%
Chance of exploitation in the next 30 days
Percentile
63%
Versus all other CVEs
30-day change
+0.01 pp
From 1.01% on 19-09-2026
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
1 foundExploit Reference (bugzilla.mozilla.org)
Maturity: unknown
Weaknesses
CWE · root causeAffected systems
Mozilla3 products
- firefox≥ unspecified, < 95 | < 95.0
- firefox_esr≥ unspecified, < 91.4.0 | < 91.4.0
- thunderbird≥ unspecified, < 91.4.0 | < 91.4.0
Debian2 products
- firefox-esr< 91.4.0esr-1
- thunderbird< 1:91.4.0-1
Ubuntu1 product
- thunderbird< 1:91.5.0+build1-0ubuntu0.18.04.1 | < 1:91.5.0+build1-0ubuntu0.20.04.1
3 vendors · 6 products
References
10 links- www.mozilla.org/security/advisories/mfsa2021-53/Vendor Advisory
- www.mozilla.org/security/advisories/mfsa2021-54/Vendor Advisory
- www.mozilla.org/security/advisories/mfsa2021-52/Vendor Advisory
- bugzilla.mozilla.org/buglist.cgi?bug_id=1393362%2C1736046%2C1736751%2C1737009%2C1739372%2C1739421ExploitIssue TrackingVendor Advisory
