CVE-2021-4140

CRITICALNVD 10.010.0—

10.0

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: 23.0%
KEV: Not listed

Published

December 22, 2022

Last Modified

April 16, 2025

Advisory Details (4)

Auto-updated May 23, 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

Patch Availability(13)

Vendor / Ecosystem	Fixed in / Patch	Released	Source
ubuntu	firefox-locale-ast (96.0+build2-0ubuntu0.21.10.1) @ impish	2026-05-25	ubuntu
ubuntu	thunderbird-locale-id (1:91.5.0+build1-0ubuntu0.20.04.1) @ focal	2026-05-25	ubuntu
ubuntu	thunderbird-locale-id (1:91.5.0+build1-0ubuntu0.21.10.1) @ impish	2026-05-25	ubuntu
redhat	firefox-0:91.5.0-1.el8_2	2022-01-12	redhat
redhat	firefox-0:91.5.0-1.el7_9	2022-01-12	redhat
redhat	thunderbird-0:91.5.0-1.el7_9	2022-01-12	redhat
redhat	firefox-0:91.5.0-1.el8_5	2022-01-12	redhat
redhat	thunderbird-0:91.5.0-1.el8_2	2022-01-12	redhat
redhat	firefox-0:91.5.0-1.el8_4	2022-01-12	redhat
redhat	thunderbird-0:91.5.0-1.el8_4	2022-01-12	redhat
redhat	thunderbird-0:91.5.0-1.el8_5	2022-01-12	redhat
redhat	firefox-0:91.5.0-1.el8_1	2022-01-12	redhat
redhat	thunderbird-0:91.5.0-1.el8_1	2022-01-12	redhat

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.

CWE-91XML Injection

All Vendor Advisories

(13)

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 10× in last 7d / 10× in last 30d)

Every time one of our enrichment pipelines (NVD, MITRE cvelistV5, EPSS, CISA KEV, GHSA, OSV, vendor advisories) ran against this CVE. Most recent first.

2026-05-27 13:39 UTCepss_batch
2026-05-27 13:39 UTCepss_batch
2026-05-26 13:43 UTCepss_batch
2026-05-26 13:42 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-26 07:17 UTCepss_batch
2026-05-25 10:03 UTCEG score recompute
2026-05-25 10:03 UTCVendor advisory
2026-05-25 10:03 UTCGHSA enrichment
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 23.0% 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

See which npm, PyPI, Go, and Maven packages are affected by CVE-2021-4140

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Is Your Infrastructure Affected by CVE-2021-4140?

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CVE-2021-4140

📅 Published

🔄 Last Modified

📋 Advisory Details (4)

Security Vulnerabilities fixed in Thunderbird 91.5 — Mozilla

Security Vulnerabilities fixed in Firefox ESR 91.5 — Mozilla

Security Vulnerabilities fixed in Firefox 96 — Mozilla

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

Patch Availability(13)

Weakness Classification(1)

All Vendor Advisories

Data Freshness Timeline

❓ Frequently asked(5)

Dependency Blast Radius

Is Your Infrastructure Affected by CVE-2021-4140?

Published

Last Modified

Advisory Details (4)

Frequently asked(5)