Is CVE-2021-4140 in the CISA KEV catalog?

No. CVE-2021-4140 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2021-4140?

CVE-2021-4140 has a CVSS v3 base score of 10.0 (CRITICAL).

What is the EPSS score for CVE-2021-4140?

CVE-2021-4140 has an EPSS score of 1.34% (68th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-4140?

Upgrade firefox on debian unstable to 96.0-1 or later. Fixes are available for 7 affected products.

CVE & CISA-KEV Catalog

CVE-2021-4140

CRITICAL

10.0

CVSS v3

NVD

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.

CWE-91

How to fix

Remediation Available

firefoxDebian

Fixed in:96.0-1CVE-2021-4140

firefoxUbuntu

Fixed in:96.0+build2-0ubuntu0.18.04.1USN-5229-1

Fixed in:96.0+build2-0ubuntu0.20.04.1USN-5229-1

thunderbirdUbuntu

Fixed in:1:91.5.0+build1-0ubuntu0.18.04.1USN-5248-1

Fixed in:1:91.5.0+build1-0ubuntu0.20.04.1USN-5248-1

DeveloperEditionWindows application

Affected:96.0Fixed in:96.0Mozilla

Mozilla Firefox (en-US)Windows application

Affected:96.0Fixed in:96.0Mozilla

Mozilla Firefox ESR (en-US)Windows application

Affected:91.5Fixed in:91.5Mozilla

Mozilla Thunderbird (en-US)Windows application

Affected:91.5Fixed in:91.5Mozilla

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityLow

Privileges RequiredNone

User InteractionNone

ScopeChanged

Impact

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

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

Exploit Intelligence

1.34%probability of exploitation in 30 days

68thpercentile

Moderate risk: more likely to be exploited than 68% of all known CVEs.

References

Patch1

bugzilla.mozilla.org

Vendor Advisory3

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2025-04-16.