Is CVE-2021-4126 in the CISA KEV catalog?

No. CVE-2021-4126 is not currently on the CISA Known Exploited Vulnerabilities list.

What is the CVSS severity of CVE-2021-4126?

CVE-2021-4126 has a CVSS v3 base score of 6.5 (MEDIUM).

What is the EPSS score for CVE-2021-4126?

CVE-2021-4126 has an EPSS score of 0.43% (35th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2021-4126?

Upgrade thunderbird on debian 11 to 1:91.4.1-1~deb11u1 or later. Fixes are available for 3 affected products.

CVE & CISA-KEV Catalog

CVE-2021-4126

MEDIUM

6.5

CVSS v3

NVD

Description

When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer added by a mailing list gateway, Thunderbird only considered the inner signed message for the signature validity. This gave the false impression that the additional contents were also covered by the digital signature. Starting with Thunderbird version 91.4.1, only the signature that belongs to the top level MIME part will be considered for the displayed status. This vulnerability affects Thunderbird < 91.4.1.

How to fix

Remediation Available

thunderbirdDebian

Fixed in:1:91.4.1-1~deb11u1CVE-2021-4126

Fixed in:1:91.4.1-1CVE-2021-4126

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

Mozilla Thunderbird (en-US)Windows application

Affected:91.4.1Fixed in:91.4.1Mozilla

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 InteractionRequired

ScopeUnchanged

Impact

ConfidentialityNone

IntegrityHigh

AvailabilityNone

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

Exploit Intelligence

0.43%probability of exploitation in 30 days

35thpercentile

Low risk: more likely to be exploited than 35% of all known CVEs.

References

Vendor Advisory2

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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.