CVE-2021-4140
CVSS v3
10
CRITICAL
EPSS Score
1.3 %
exploit probability, as of 2026-09-25
CISA KEV
No
known exploited
Exploitation
—
SSVC status
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.
Technical details
- Published
- 2022-12-22
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.
Is CVE-2021-4140 actively exploited?
Active exploitation of CVE-2021-4140 has not been confirmed. Its EPSS score was 1.3% on 2026-09-25, the estimated probability of exploitation in the next 30 days.
What is the CVSS score for CVE-2021-4140?
CVE-2021-4140 has a CVSS v3 base score of 10 (CRITICAL severity).
Is CVE-2021-4140 affecting your environment?
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