CVE-2021-4140

Name: CVE-2021-4140
Creator: NVD / NIST
Published: 2022-12-22T20:15:12.25+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 10/10EPSS 1.34%

Last modified Jun 17, 2026

CVE-2021-4140 is a critical-severity vulnerability rated 10/10 on the CVSS scale. 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.. EPSS estimates a 1.34% chance of exploitation in the next 30 days.

Description

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.

Metrics

CVSS 3.1

10/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

EPSS Probability

1.34%

67.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Mozilla	Firefox	< 96.0
Mozilla	Firefox Esr	< 91.5
Mozilla	Thunderbird	< 91.5

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1746720Exploit, Issue Tracking, Patch, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-01/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-02/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-03/Vendor Advisory
https://bugzilla.mozilla.org/show_bug.cgi?id=1746720Exploit, Issue Tracking, Patch, Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-01/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-02/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2022-03/Vendor Advisory

Timeline

Published: Dec 22, 2022
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-4140?

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.

How severe is CVE-2021-4140?

CVE-2021-4140 has a CVSS score of 10/10 (CRITICAL severity). The EPSS model estimates a 1.34% probability of exploitation in the next 30 days.

How do I fix CVE-2021-4140?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-4140?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST