CVE-2021-4129

CRITICALNVD 9.89.8—
EchelonGraph scoreMEDIUM confidence

Score 9.8 from GitHub Security Advisory (severity: CRITICAL) published 2022-12-22. NVD baseline CVSS 9.8; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Trending — 3 sources updated this week
9.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 1%CVSS: 9.8Exploit: NoneExposed: 0

A fix is available — apply it.

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 v3
9.8
EG Score
9.8(medium)
EG Risk
60(Track)
EG Risk 60/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity98% × 45%
Exploitation1% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS
59.6%
KEV
Not listed

Published

December 22, 2022

Last Modified

April 16, 2025

Advisory Details (4)

Auto-updated Jul 12, 2026
No patch confirmed yet.
generic

Security Vulnerabilities fixed in Thunderbird 91.4.0 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-54/
generic

Security Vulnerabilities fixed in Firefox ESR 91.4.0 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-53/
generic

Security Vulnerabilities fixed in Firefox 95 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-52/
generic

Client Challenge

https://bugzilla.mozilla.org/buglist.cgi?bug_id=1393362%2C1736046%2C1736751%2C1737009%2C1739372%2C1739421

Vendor Advisories for CVE-2021-4129(2)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(12)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.21.10.1) @ impish2026-05-25ubuntu
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.20.04.1) @ focal2026-05-25ubuntu
redhatthunderbird-0:91.4.0-2.el8_22021-12-09redhat
redhatthunderbird-0:91.4.0-2.el8_52021-12-09redhat
redhatthunderbird-0:91.4.0-2.el8_42021-12-09redhat
redhatthunderbird-0:91.4.0-3.el7_92021-12-09redhat
redhatthunderbird-0:91.4.0-2.el8_12021-12-09redhat
redhatfirefox-0:91.4.0-1.el8_42021-12-08redhat
redhatfirefox-0:91.4.0-1.el8_52021-12-08redhat
redhatfirefox-0:91.4.0-1.el8_22021-12-08redhat
redhatfirefox-0:91.4.0-1.el8_12021-12-08redhat
redhatfirefox-0:91.4.0-1.el7_92021-12-08redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Additional Vendor Advisories

(10)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 13× in last 7d / 48× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 102 total refreshes for this CVE.

  1. 2026-07-23 02:04 UTCEG score recompute
  2. 2026-07-22 22:50 UTCEG score recompute
  3. 2026-07-22 14:06 UTCEPSS rescore
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  43. 2026-06-10 21:17 UTCOSV refresh
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  74. 2026-05-25 10:03 UTCEG score recompute
  75. 2026-05-25 10:03 UTCVendor advisory

Related CVEs(same vendor + same CWE)

Frequently asked(5)

What is CVE-2021-4129?
CVE-2021-4129 is a critical vulnerability published on December 22, 2022. 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…
When was CVE-2021-4129 disclosed?
CVE-2021-4129 was first published in the National Vulnerability Database on December 22, 2022, with the most recent update on April 16, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-4129 actively exploited?
CVE-2021-4129 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 59.6% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-4129?
CVE-2021-4129 has a CVSS v3 base score of 9.8 (NVD).
How do I remediate CVE-2021-4129?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-4129, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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