CVE-2021-4140

CRITICALNVD 10.010.0—
EchelonGraph scoreMEDIUM confidence

Score 10.0 from GitHub Security Advisory (severity: CRITICAL) published 2022-12-22. NVD baseline CVSS 10.0; sources differ by 0.0.

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
Elevated
10.0
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 1%CVSS: 10.0Exploit: NoneExposed: 0

A fix is available — apply it.

It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.

CVSS v3
10.0
EG Score
10.0(medium)
EPSS
68.0%
KEV
Not listed

Published

December 22, 2022

Last Modified

April 16, 2025

Advisory Details (4)

Auto-updated Jul 11, 2026
No patch confirmed yet.
generic

Security Vulnerabilities fixed in Thunderbird 91.5 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2022-03/
generic

Security Vulnerabilities fixed in Firefox ESR 91.5 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2022-02/
generic

Security Vulnerabilities fixed in Firefox 96 — Mozilla

https://www.mozilla.org/security/advisories/mfsa2022-01/
generic

1746720 - (CVE-2021-4140) Meta refresh via XSLT stylesheet bypasses <iframe sandbox> attribute restrictions

https://bugzilla.mozilla.org/show_bug.cgi?id=1746720

Vendor Advisories for CVE-2021-4140(2)

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

Patch Availability(13)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntufirefox-locale-ast (96.0+build2-0ubuntu0.21.10.1) @ impish2026-05-25ubuntu
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.20.04.1) @ focal2026-05-25ubuntu
ubuntuthunderbird-locale-id (1:91.5.0+build1-0ubuntu0.21.10.1) @ impish2026-05-25ubuntu
redhatfirefox-0:91.5.0-1.el8_22022-01-12redhat
redhatfirefox-0:91.5.0-1.el7_92022-01-12redhat
redhatthunderbird-0:91.5.0-1.el7_92022-01-12redhat
redhatfirefox-0:91.5.0-1.el8_52022-01-12redhat
redhatthunderbird-0:91.5.0-1.el8_22022-01-12redhat
redhatfirefox-0:91.5.0-1.el8_42022-01-12redhat
redhatthunderbird-0:91.5.0-1.el8_42022-01-12redhat
redhatthunderbird-0:91.5.0-1.el8_52022-01-12redhat
redhatfirefox-0:91.5.0-1.el8_12022-01-12redhat
redhatthunderbird-0:91.5.0-1.el8_12022-01-12redhat

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(1)

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

Additional Vendor Advisories

(11)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 11× in last 7d / 45× 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.

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

Frequently asked(5)

What is CVE-2021-4140?
CVE-2021-4140 is a critical vulnerability published on December 22, 2022. It was possible to construct specific XSLT markup that would be able to bypass an iframe sandbox. This vulnerability affects Firefox ESR < 91.5, Firefox < 96, and Thunderbird < 91.5.
When was CVE-2021-4140 disclosed?
CVE-2021-4140 was first published in the National Vulnerability Database on December 22, 2022, with the most recent update on April 16, 2025. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2021-4140 actively exploited?
CVE-2021-4140 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 68.0% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2021-4140?
CVE-2021-4140 has a CVSS v3 base score of 10.0 (NVD).
How do I remediate CVE-2021-4140?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2021-4140, 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

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