CVE-2026-84121

CRITICALPre-NVD 9.69.6—
EchelonGraph scoreHIGH confidence

Score 9.6 from GitHub Security Advisory (severity: CRITICAL) published 2026-09-01. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 9.6; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: cisa-adp, epss, ghsa
Trending — Patch released this week
9.6EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 9.6Exploit: None knownExposed: 0

A fix is available — apply it.

Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

CVSS v3
9.6
EG Score
9.6(high)
EG Risk
43(Track)
EG Risk 43/100SSVC: 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
Severity96% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
22%
KEV
Not listed

Published

September 1, 2026

Last Modified

September 2, 2026

Advisory Details (8)

Auto-updated Sep 22, 2026
No patch confirmed yet.
generic

Security Vulnerabilities fixed in Thunderbird 153.2 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-88/
generic

Security Vulnerabilities fixed in Thunderbird 140.15 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-87/
generic

Security Vulnerabilities fixed in Thunderbird 155 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-86/
generic

Security Vulnerabilities fixed in Firefox ESR 153.2 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-85/
generic

Security Vulnerabilities fixed in Firefox ESR 140.15 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-84/
generic

Security Vulnerabilities fixed in Firefox ESR 115.40 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-83/
generic

Security Vulnerabilities fixed in Firefox 155 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-82/
generic

Access Denied

https://bugzilla.mozilla.org/show_bug.cgi?id=2059018

Vendor Advisories for CVE-2026-84121(1)

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

Patch Availability(1)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatthunderbird-0:140.16.0-1.el9_82026-09-24redhat

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.

Affected Packages

(6 across 3 ecosystems)
Debian:12(2)
PackageVulnerable rangeFixed inDependents
firefox-esr102.11.0esr-1 ... 140.9.1esr-1~deb13u1 (198 versions)140.15.0esr-1~deb12u1—
thunderbird1:102.11.0-1 ... 1:140.9.1esr-1~deb13u1 (227 versions)1:140.15.0esr-1~deb12u1—
Debian:13(2)
PackageVulnerable rangeFixed inDependents
firefox-esr128.13.0esr-1 ... 140.9.1esr-1~deb13u1 (74 versions)140.15.0esr-1~deb13u1—
thunderbird1:128.13.0esr-1 ... 1:140.9.1esr-1~deb13u1 (83 versions)1:140.15.0esr-1~deb13u1—
Debian:14(2)
PackageVulnerable rangeFixed inDependents
firefox-esr128.13.0esr-1 ... 140.9.1esr-1~deb13u1 (75 versions)140.15.0esr-1—
thunderbird1:128.13.0esr-1 ... 1:153.2.0esr-1 (93 versions)——

Weakness Classification(1)

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

All Vendor Advisories

(1)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 61× in last 7d / 199× 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 199 total refreshes for this CVE.

  1. 2026-09-25 09:49 UTCVendor advisory
  2. 2026-09-25 09:49 UTCGHSA enrichment
  3. 2026-09-25 05:54 UTCVendor advisory
  4. 2026-09-25 05:54 UTCGHSA enrichment
  5. 2026-09-25 01:59 UTCVendor advisory
  6. 2026-09-25 01:59 UTCGHSA enrichment
  7. 2026-09-24 22:03 UTCVendor advisory
  8. 2026-09-24 22:03 UTCGHSA enrichment
  9. 2026-09-24 17:12 UTCEG score recompute
  10. 2026-09-24 17:12 UTCVendor advisory
  11. 2026-09-24 17:12 UTCGHSA enrichment
  12. 2026-09-24 14:04 UTCEPSS rescore
  13. 2026-09-24 13:16 UTCVendor advisory
  14. 2026-09-24 13:16 UTCGHSA enrichment
  15. 2026-09-24 09:20 UTCGHSA enrichment
  16. 2026-09-24 05:25 UTCGHSA enrichment
  17. 2026-09-24 01:29 UTCGHSA enrichment
  18. 2026-09-23 21:34 UTCEG score recompute
  19. 2026-09-23 21:34 UTCGHSA enrichment
  20. 2026-09-23 17:54 UTCEPSS rescore
  21. 2026-09-23 17:39 UTCGHSA enrichment
  22. 2026-09-23 13:44 UTCGHSA enrichment
  23. 2026-09-23 09:48 UTCGHSA enrichment
  24. 2026-09-23 05:53 UTCGHSA enrichment
  25. 2026-09-23 01:58 UTCGHSA enrichment
Show 75 more
  1. 2026-09-22 22:02 UTCGHSA enrichment
  2. 2026-09-22 18:07 UTCEG score recompute
  3. 2026-09-22 18:07 UTCGHSA enrichment
  4. 2026-09-22 16:01 UTCEPSS rescore
  5. 2026-09-22 13:10 UTCGHSA enrichment
  6. 2026-09-22 09:14 UTCGHSA enrichment
  7. 2026-09-22 05:01 UTCGHSA enrichment
  8. 2026-09-22 01:04 UTCGHSA enrichment
  9. 2026-09-21 21:09 UTCEG score recompute
  10. 2026-09-21 21:09 UTCGHSA enrichment
  11. 2026-09-21 21:09 UTCEPSS rescore
  12. 2026-09-21 17:13 UTCGHSA enrichment
  13. 2026-09-21 12:59 UTCGHSA enrichment
  14. 2026-09-21 09:03 UTCGHSA enrichment
  15. 2026-09-21 05:08 UTCGHSA enrichment
  16. 2026-09-21 00:41 UTCGHSA enrichment
  17. 2026-09-20 20:45 UTCEG score recompute
  18. 2026-09-20 20:45 UTCGHSA enrichment
  19. 2026-09-20 20:16 UTCEPSS rescore
  20. 2026-09-20 16:50 UTCGHSA enrichment
  21. 2026-09-20 12:46 UTCGHSA enrichment
  22. 2026-09-20 08:50 UTCGHSA enrichment
  23. 2026-09-20 04:54 UTCGHSA enrichment
  24. 2026-09-20 00:57 UTCGHSA enrichment
  25. 2026-09-19 21:01 UTCGHSA enrichment
  26. 2026-09-19 17:04 UTCEG score recompute
  27. 2026-09-19 17:04 UTCGHSA enrichment
  28. 2026-09-19 13:09 UTCGHSA enrichment
  29. 2026-09-19 09:14 UTCGHSA enrichment
  30. 2026-09-19 05:17 UTCGHSA enrichment
  31. 2026-09-19 01:21 UTCGHSA enrichment
  32. 2026-09-18 21:25 UTCEG score recompute
  33. 2026-09-18 21:25 UTCGHSA enrichment
  34. 2026-09-18 19:28 UTCEPSS rescore
  35. 2026-09-18 16:23 UTCGHSA enrichment
  36. 2026-09-18 12:27 UTCGHSA enrichment
  37. 2026-09-18 08:31 UTCGHSA enrichment
  38. 2026-09-18 04:36 UTCGHSA enrichment
  39. 2026-09-18 00:41 UTCGHSA enrichment
  40. 2026-09-17 20:46 UTCEG score recompute
  41. 2026-09-17 20:46 UTCGHSA enrichment
  42. 2026-09-17 19:32 UTCEPSS rescore
  43. 2026-09-17 14:51 UTCGHSA enrichment
  44. 2026-09-17 10:56 UTCGHSA enrichment
  45. 2026-09-17 07:00 UTCGHSA enrichment
  46. 2026-09-17 03:04 UTCGHSA enrichment
  47. 2026-09-16 23:01 UTCGHSA enrichment
  48. 2026-09-16 19:05 UTCGHSA enrichment
  49. 2026-09-16 15:10 UTCEG score recompute
  50. 2026-09-16 15:10 UTCGHSA enrichment
  51. 2026-09-16 14:09 UTCEPSS rescore
  52. 2026-09-16 11:14 UTCGHSA enrichment
  53. 2026-09-16 07:19 UTCEG score recompute
  54. 2026-09-16 07:19 UTCGHSA enrichment
  55. 2026-09-16 05:15 UTCEPSS rescore
  56. 2026-09-16 03:23 UTCGHSA enrichment
  57. 2026-09-15 23:26 UTCGHSA enrichment
  58. 2026-09-15 19:03 UTCGHSA enrichment
  59. 2026-09-15 15:06 UTCGHSA enrichment
  60. 2026-09-15 11:02 UTCGHSA enrichment
  61. 2026-09-15 06:27 UTCEG score recompute
  62. 2026-09-15 06:26 UTCGHSA enrichment
  63. 2026-09-15 03:12 UTCEPSS rescore
  64. 2026-09-15 03:12 UTCEPSS rescore
  65. 2026-09-15 02:31 UTCGHSA enrichment
  66. 2026-09-14 22:35 UTCGHSA enrichment
  67. 2026-09-14 18:39 UTCGHSA enrichment
  68. 2026-09-14 14:33 UTCGHSA enrichment
  69. 2026-09-14 10:37 UTCGHSA enrichment
  70. 2026-09-14 06:41 UTCGHSA enrichment
  71. 2026-09-14 02:46 UTCGHSA enrichment
  72. 2026-09-13 22:50 UTCGHSA enrichment
  73. 2026-09-13 18:55 UTCEG score recompute
  74. 2026-09-13 18:55 UTCGHSA enrichment
  75. 2026-09-13 14:55 UTCGHSA enrichment

Related CVEs(same vendor + same CWE)

Frequently asked(5)

What is CVE-2026-84121?
CVE-2026-84121 is a critical vulnerability published on September 1, 2026. Sandbox escape due to use-after-free in the DOM: Security component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, Firefox ESR 153.2, Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
When was CVE-2026-84121 disclosed?
CVE-2026-84121 was first published in the National Vulnerability Database on September 1, 2026, with the most recent update on September 2, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-84121 actively exploited?
CVE-2026-84121 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 77.8% of all scored CVEs.
What is the CVSS score of CVE-2026-84121?
CVE-2026-84121 has a CVSS v3.1 base score of 9.6 (CISA-ADP / Vulnrichment enrichment; NVD's own analysis pending).
How do I remediate CVE-2026-84121?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-84121, 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

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-84121

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