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

TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works

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
69thpercentile

Moderate risk: more likely to be exploited than 69% of all known CVEs.

References

Related Vulnerabilities

Other CWE-91 vulnerabilities, ordered by exploit likelihood. View all

CVESeverityCVSSEPSSExploitedFix
CVE-2020-0646Critical9.899%KEV-
CVE-2023-27253High8.890%--
CVE-2023-46214High8.089%-Fix
CVE-2024-53675High7.384%-Fix
CVE-2024-53674High7.347%-Fix
CVE-2023-43187Critical9.845%-Fix

Common questions

How do I fix CVE-2021-4140?

Published advisories record a fix for 7 affected products. The "How to fix" section on this page lists the fixed version and source advisory for each one, so apply the entry matching what you actually run.

Is CVE-2021-4140 being actively exploited?

Not that we know of. CVE-2021-4140 is not in the CISA Known Exploited Vulnerabilities catalog. Its EPSS score of 1.3% is the estimated probability that it will be exploited in the next 30 days. That is higher than 69% of all scored CVEs.

How severe is CVE-2021-4140?

CVE-2021-4140 has a CVSS v3 base score of 10.0, rated critical. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.

What does CVE-2021-4140 affect?

Published advisories record a fix for firefox (Debian), firefox (Ubuntu), thunderbird (Ubuntu), DeveloperEdition (Windows application), and 3 more. Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.

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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.