What’s Vulnerable › Vulnerabilities

CVE-2021-4140

• Severity: critical (CVSS 10.0 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.4% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2022-12-22 · updated 2026-06-17
• Weakness: CWE-91

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.

Affected products

• Mozilla Firefox: before 96.0 — fixed in 96.0
• Mozilla Firefox ESR: before 91.5 — fixed in 91.5
• Mozilla Thunderbird: before 91.5 — fixed in 91.5

How to fix

Upgrade to a fixed version: Mozilla Firefox 96.0; Mozilla Firefox ESR 91.5; Mozilla Thunderbird 91.5. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1746720
• https://www.mozilla.org/security/advisories/mfsa2022-01/
• https://www.mozilla.org/security/advisories/mfsa2022-02/
• https://www.mozilla.org/security/advisories/mfsa2022-03/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1746720
• https://www.mozilla.org/security/advisories/mfsa2022-01/
• https://www.mozilla.org/security/advisories/mfsa2022-02/
• https://www.mozilla.org/security/advisories/mfsa2022-03/

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