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
10.0EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1%CVSS: 10.0Exploit: Elevated riskExposed: 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)
EG Risk
76(Attend)
EG Risk 76/100SSVC: Attend

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
Severity100% × 45%
Exploitation40% × 40%
Automatability100% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS PROB
1%
EPSS %ILE
69%
KEV
Not listed

Published

December 22, 2022

Last Modified

April 16, 2025

Advisory Details (4)

Auto-updated Aug 22, 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.

Affected Packages

(12 across 6 ecosystems)
Debian:10(2)
PackageVulnerable rangeFixed inDependents
firefox-esr60.7.2esr-1 ... 91.5.0esr-1~deb9u1 (125 versions)91.5.0esr-1~deb10u1—
thunderbird1:60.7.2-1 ... 1:91.5.0-1~deb9u1 (157 versions)1:91.5.0-2~deb10u1—
Debian:11(2)
PackageVulnerable rangeFixed inDependents
firefox-esr78.12.0esr-1 ... 91.5.0esr-1~deb9u1 (23 versions)91.5.0esr-1~deb11u1—
thunderbird1:78.12.0-1 ... 1:91.5.0-2~deb10u1 (34 versions)1:91.5.0-2~deb11u1—
Debian:12(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—91.5.0esr-1—
thunderbird—1:91.5.0-1—
Debian:13(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—91.5.0esr-1—
thunderbird—1:91.5.0-1—
Debian:14(2)
PackageVulnerable rangeFixed inDependents
firefox-esr—91.5.0esr-1—
thunderbird—1:91.5.0-1—
Debian:9(2)
PackageVulnerable rangeFixed inDependents
firefox-esr45.9.0esr-1 ... 91.4.1esr-1~deb9u1 (232 versions)91.5.0esr-1~deb9u1—
thunderbird1.5-4 ... 1:91.4.1-1~deb9u1 (237 versions)1:91.5.0-1~deb9u1—

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 5× in last 7d / 28× 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 135 total refreshes for this CVE.

  1. 2026-08-25 13:46 UTCEPSS rescore
  2. 2026-08-24 14:13 UTCEPSS rescore
  3. 2026-08-21 23:46 UTCEPSS rescore
  4. 2026-08-20 22:52 UTCEPSS rescore
  5. 2026-08-19 17:01 UTCEPSS rescore
  6. 2026-08-18 13:45 UTCEPSS rescore
  7. 2026-08-18 07:22 UTCOSV refresh
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  21. 2026-08-04 10:34 UTCEPSS rescore
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  23. 2026-08-03 02:27 UTCOSV refresh
  24. 2026-08-02 02:24 UTCEPSS rescore
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  1. 2026-07-30 16:25 UTCEPSS rescore
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  8. 2026-07-23 14:16 UTCEPSS rescore
  9. 2026-07-23 02:04 UTCEG score recompute
  10. 2026-07-22 22:51 UTCEG score recompute
  11. 2026-07-22 14:06 UTCEPSS rescore
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  21. 2026-07-16 11:49 UTCOSV refresh
  22. 2026-07-15 16:55 UTCEPSS rescore
  23. 2026-07-15 01:58 UTCEPSS rescore
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  25. 2026-07-13 22:27 UTCEPSS rescore
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  28. 2026-07-12 05:44 UTCEPSS rescore
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  30. 2026-07-09 19:07 UTCEPSS rescore
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  43. 2026-06-30 23:20 UTCEPSS rescore
  44. 2026-06-29 14:04 UTCEPSS rescore
  45. 2026-06-29 14:04 UTCEPSS rescore
  46. 2026-06-28 10:49 UTCOSV refresh
  47. 2026-06-28 04:54 UTCEPSS rescore
  48. 2026-06-28 04:54 UTCEPSS rescore
  49. 2026-06-27 03:07 UTCEPSS rescore
  50. 2026-06-27 03:06 UTCEPSS rescore
  51. 2026-06-25 13:48 UTCEPSS rescore
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  71. 2026-06-13 22:58 UTCEPSS rescore
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  73. 2026-06-12 23:10 UTCEPSS rescore
  74. 2026-06-11 13:58 UTCEPSS rescore
  75. 2026-06-10 22:16 UTCEPSS rescore

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 1% probability of exploitation in the next 30 days, which ranks it in the top 30.8% of all scored CVEs.
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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