CVE-2021-23960

HIGHNVD 8.88.8—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 8.8 under NVD CVSS v3. EPSS exploit probability: 1.3% (69th percentile of EPSS-scored CVEs). GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
8.8EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1.3%CVSS: 8.8Exploit: None knownExposed services: Not assessed

A fix is available — apply it.

Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

CVSS v3
8.8
EG Score
8.8HIGHmedium confidence
EG Risk
45
EG Risk 45/100CISA SSVC

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
Severity88% × 45%
Exploitation1% × 40%
Automatability30% × 15%
CISA SSVC: Track at low or medium mission impact; Track or Attend at high (mission-essential systems).
Action: A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.
EPSS PROB
1.3%
EPSS %ILE
69th
KEV
Not listed

CISA SSVCTrack at low or medium mission impact; Track or Attend at high (mission-essential systems).

A fix is available. Apply it within your standard update timelines at low or medium mission impact and sooner than that at high.

Exploitation none (no KEV listing, exploit record or EPSS ≥ 50%) · Automatable unknown (not published for this CVE) · Technical impact partial (CVSS below 9.0). Mission impact is CISA's Mission & Well-being decision point, and only you can judge it: high means the affected system is essential to your organisation's mission, or its compromise could cause irreversible harm to people. CISA's decision table

Published

February 26, 2021

Last Modified

November 21, 2024

Advisory Details (4)

Auto-updated Aug 4, 2026
Patch available.
generic

Security Vulnerabilities fixed in Thunderbird 78.7 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-05/
generic

Security Vulnerabilities fixed in Firefox ESR 78.7 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-04/
generic

Security Vulnerabilities fixed in Firefox 85 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2021-03/
generic

1675755 - (CVE-2021-23960) Crash on GC after redeclare let or const on the web console [js::gc::Cell::runtimeFromAnyThread (this=0x4b4b4b4b4b4b4b4b)]

https://bugzilla.mozilla.org/show_bug.cgi?id=1675755

Patch Availability(10)

Vendor / EcosystemFix (by version range) / PatchReleasedSource
ubuntufirefox-mozsymbols (85.0+build1-0ubuntu0.18.04.1) @ bionic2026-05-26ubuntu
ubuntuxul-ext-lightning (1:78.7.1+build1-0ubuntu0.20.10.4) @ groovy2026-05-26ubuntu
redhatthunderbird-0:78.7.0-1.el8_12021-02-03redhat
redhatthunderbird-0:78.7.0-1.el8_22021-01-28redhat
redhatfirefox-0:78.7.0-2.el7_92021-01-28redhat
redhatthunderbird-0:78.7.0-1.el7_92021-01-28redhat
redhatthunderbird-0:78.7.0-1.el8_32021-01-28redhat
redhatfirefox-0:78.7.0-2.el8_32021-01-27redhat
redhatfirefox-0:78.7.0-2.el8_12021-01-27redhat
redhatfirefox-0:78.7.0-2.el8_22021-01-27redhat

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.

Affected Packages

(12 across 6 ecosystems)
Debian:10(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr60.7.2esr-1 ... 78.7.0esr-1~deb9u1 (87 versions)
  • every version up to 78.7.0esr-1~deb10u1: fixed in 78.7.0esr-1~deb10u1
—
thunderbird1:60.7.2-1 ... 1:78.7.0-1~deb9u1 (105 versions)
  • every version up to 1:78.7.0-1~deb10u1: fixed in 1:78.7.0-1~deb10u1
—
Debian:11(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.7.0esr-1: fixed in 78.7.0esr-1
—
thunderbird—
  • every version up to 1:78.7.0-1: fixed in 1:78.7.0-1
—
Debian:12(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.7.0esr-1: fixed in 78.7.0esr-1
—
thunderbird—
  • every version up to 1:78.7.0-1: fixed in 1:78.7.0-1
—
Debian:13(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.7.0esr-1: fixed in 78.7.0esr-1
—
thunderbird—
  • every version up to 1:78.7.0-1: fixed in 1:78.7.0-1
—
Debian:14(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr—
  • every version up to 78.7.0esr-1: fixed in 78.7.0esr-1
—
thunderbird—
  • every version up to 1:78.7.0-1: fixed in 1:78.7.0-1
—
Debian:9(2)
PackageVulnerable rangeFix by version rangeDependents
firefox-esr45.9.0esr-1 ... 78.6.1esr-1~deb9u1 (194 versions)
  • every version up to 78.7.0esr-1~deb9u1: fixed in 78.7.0esr-1~deb9u1
—
thunderbird1.5-4 ... 1:78.6.1-1 (186 versions)
  • every version up to 1:78.7.0-1~deb9u1: fixed in 1:78.7.0-1~deb9u1
—

All Vendor Advisories

(10)

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 8× in last 7d / 34× 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 179 total refreshes for this CVE.

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

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Frequently asked(5)

What is CVE-2021-23960?
CVE-2021-23960 is a high vulnerability published on February 26, 2021. Performing garbage collection on re-declared JavaScript variables resulted in a user-after-poison, and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.
When was CVE-2021-23960 disclosed?
CVE-2021-23960 was first published on February 26, 2021, with the most recent update on November 21, 2024. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-23960 actively exploited?
CVE-2021-23960 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1.3% probability of exploitation in the next 30 days (69th percentile of EPSS-scored CVEs).
What is the CVSS score of CVE-2021-23960?
CVE-2021-23960 has a CVSS v3 base score of 8.8 (NVD).
How do I remediate CVE-2021-23960?
A fix for CVE-2021-23960 is available: update to the fixed version the vendor names in its advisory.

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