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Fencer Vulns
medium 6.5

CVE-2021-4126 json

When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer adde…

published 2022-12-22 3 y ago modified 2026-06-17 4 mo ago also EUVD-2021-34000

cvss
6.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 40%
public exploits
0
none known

Summary

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.

Patch / fix

  • Mozilla Thunderbird 91.4.1
  • thunderbird Debian 14, Debian 13, Debian 12 1:91.4.1-1

debian advisories DEBIAN-CVE-2021-4126

  • thunderbird Ubuntu 18.04 LTS 1:91.5.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:91.5.0+build1-0ubuntu0.20.04.1
  • thunderbird Ubuntu 22.04 LTS 1:91.5.0+build1-0ubuntu1

ubuntu advisories UBUNTU-CVE-2021-4126

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • MozillaThunderbird SLE Workstation Extension 15 SP3 91.4.1-8.48.1

suse advisories SUSE-SU-2022:0058-1

  • MozillaThunderbird openSUSE Tumbleweed 91.4.1-1.1
  • MozillaThunderbird openSUSE Leap 15.3 91.4.1-8.48.1
  • zabbix Package Hub 15 SP3, openSUSE Leap 15.3 4.0.38-bp153.2.3.1

opensuse advisories openSUSE-SU-2024:11698-1openSUSE-SU-2022:0058-1

  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.4.1-r0

alpine advisories ALPINE-CVE-2021-4126

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
18 mo ago

Exploit prediction

0.46% chance of exploitation in the next 30 days, higher than 40% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
CNA 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Affected products

  • Mozilla Thunderbird

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.