CVE-2021-23954

HIGHNVD 8.88.8—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 8.8 under NVD CVSS v3. EPSS exploit probability: 1.1%, top 38% of all CVEs by exploit prediction. 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%CVSS: 8.8Exploit: None knownExposed: 0

A fix is available — apply it.

Using the new logical assignment operators in a JavaScript switch statement could have caused a type confusion, leading to a memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

CVSS v3
8.8
EG Score
8.8(medium)
EG Risk
45(Track)
EG Risk 45/100SSVC: 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
Severity88% × 45%
Exploitation1% × 40%
Automatability30% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
1%
EPSS %ILE
66%
KEV
Not listed

Published

February 26, 2021

Last Modified

November 21, 2024

Advisory Details (4)

Auto-updated Aug 4, 2026
No patch confirmed yet.
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

1684020 - (CVE-2021-23954) Assertion failure: next == JSOp::CheckThis || next == JSOp::CheckReturn || next == JSOp::CheckThisReinit || next == JSOp::CheckLexical, at vm/Interpreter.cpp:3715 or Assertion failure: v.isSymbol() || v.isBigInt(), at jsnum.cpp:1944

https://bugzilla.mozilla.org/show_bug.cgi?id=1684020

Patch Availability(10)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntufirefox-mozsymbols (85.0+build1-0ubuntu0.20.10.1) @ groovy2026-05-26ubuntu
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.el7_92021-01-28redhat
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.el8_32021-01-28redhat
redhatfirefox-0:78.7.0-2.el8_22021-01-27redhat
redhatfirefox-0:78.7.0-2.el8_32021-01-27redhat
redhatfirefox-0:78.7.0-2.el8_12021-01-27redhat

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 rangeFixed inDependents
firefox-esr60.7.2esr-1 ... 78.7.0esr-1~deb9u1 (87 versions)78.7.0esr-1~deb10u1—
thunderbird1:60.7.2-1 ... 1:78.7.0-1~deb9u1 (105 versions)1:78.7.0-1~deb10u1—
Debian:11(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—78.7.0esr-1—
thunderbird—1:78.7.0-1—
Debian:12(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—78.7.0esr-1—
thunderbird—1:78.7.0-1—
Debian:13(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—78.7.0esr-1—
thunderbird—1:78.7.0-1—
Debian:14(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—78.7.0esr-1—
thunderbird—1:78.7.0-1—
Debian:9(2)
PackageVulnerable rangeFixed inDependents
firefox-esr45.9.0esr-1 ... 78.6.1esr-1~deb9u1 (194 versions)78.7.0esr-1~deb9u1—
thunderbird1.5-4 ... 1:78.6.1-1 (186 versions)1:78.7.0-1~deb9u1—

Weakness Classification(1)

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

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 7× in last 7d / 30× 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 161 total refreshes for this CVE.

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

What is CVE-2021-23954?
CVE-2021-23954 is a high vulnerability published on February 26, 2021. Using the new logical assignment operators in a JavaScript switch statement could have caused a type confusion, leading to a memory corruption and a potentially exploitable crash. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.
When was CVE-2021-23954 disclosed?
CVE-2021-23954 was first published in the National Vulnerability Database 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-2021-23954 actively exploited?
CVE-2021-23954 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1% probability of exploitation in the next 30 days, which ranks it in the top 33.6% of all scored CVEs.
What is the CVSS score of CVE-2021-23954?
CVE-2021-23954 has a CVSS v3 base score of 8.8 (NVD).
How do I remediate CVE-2021-23954?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-23954, 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-2021-23954

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