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
9.6
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 9.6Exploit: NoneExposed: 0

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
9.6(medium)
EPSS
33.6%
KEV
Not listed

Published

May 19, 2026

Last Modified

May 20, 2026

Advisory Details (5)

Auto-updated May 19, 2026
No patch confirmed yet.
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

Access Denied

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 / EcosystemFixed in / PatchReleasedSource
redhatthunderbird-0:140.11.0-1.el9_62026-06-17redhat
redhatthunderbird-0:140.11.0-1.el10_02026-06-17redhat
redhatthunderbird-0:140.11.0-1.el8_62026-06-17redhat
redhatthunderbird-0:140.11.0-1.el9_42026-06-16redhat
redhatthunderbird-0:140.11.0-1.el9_22026-06-16redhat
redhatthunderbird-0:140.11.0-1.el8_42026-06-16redhat
redhatthunderbird-0:140.11.0-1.el8_82026-06-16redhat
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 9× in last 7d / 226× 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 350 total refreshes for this CVE.

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

Related CVEs(same vendor + same CWE)

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 in the National Vulnerability Database 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 33.6% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
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?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-8959, 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.

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