CVE-2026-8959

CRITICALNVD 9.69.6—
EchelonGraph scoreMEDIUM confidence

Score 9.6 from GitHub Security Advisory (severity: CRITICAL) published 2026-05-19. NVD baseline CVSS 9.6; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
9.6EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0.4%CVSS: 9.6Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

CVSS v3
9.6
EG Score
9.6CRITICALmedium confidence
EG Risk
43
EG Risk 43/100

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%
EPSS PROB
0.4%
EPSS %ILE
37th
KEV
Not listed

Published

May 19, 2026

Last Modified

May 20, 2026

Advisory Details (5)

Auto-updated Oct 2, 2026
Patch available.
generic

Security Vulnerabilities fixed in Thunderbird 140.11 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-51/
generic

Security Vulnerabilities fixed in Thunderbird 151 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-50/
generic

Security Vulnerabilities fixed in Firefox ESR 140.11 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-48/
generic

Security Vulnerabilities fixed in Firefox 151 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2026-46/
generic

2034754 - (CVE-2026-8959) Heap-Buffer-Overflow on Parent Process (Drag and Drop feature)

https://bugzilla.mozilla.org/show_bug.cgi?id=2034754

Vendor Advisories for CVE-2026-8959(11)

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

Patch Availability(10)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
redhatthunderbird-0:140.11.0-1.el10_02026-06-17redhat
redhatthunderbird-0:140.11.0-1.el9_62026-06-17redhat
redhatthunderbird-0:140.11.0-1.el8_62026-06-17redhat
redhatthunderbird-0:140.11.0-1.el8_82026-06-16redhat
redhatthunderbird-0:140.11.0-1.el8_42026-06-16redhat
redhatthunderbird-0:140.11.0-1.el9_22026-06-16redhat
redhatthunderbird-0:140.11.0-1.el9_42026-06-16redhat
redhatthunderbird-0:140.11.0-1.el8_102026-06-03redhat
redhatthunderbird-0:140.11.0-1.el10_22026-06-01redhat
redhatthunderbird-0:140.11.0-1.el9_82026-05-27redhat

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(3)

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

Data Freshness Timeline

(refreshed 6× in last 7d / 27× 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 490 total refreshes for this CVE.

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

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Frequently asked(5)

What is CVE-2026-8959?
CVE-2026-8959 is a critical vulnerability published on May 19, 2026. Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.
When was CVE-2026-8959 disclosed?
CVE-2026-8959 was first published on May 19, 2026, with the most recent update on May 20, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-8959 actively exploited?
CVE-2026-8959 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0.4% probability of exploitation in the next 30 days (37th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2026-8959?
CVE-2026-8959 has a CVSS v3 base score of 9.6 (NVD).
How do I remediate CVE-2026-8959?
A fix for CVE-2026-8959 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-8959 are linked in the Vendor Advisories panel on this page.

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