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Fencer Intel
medium 6.8

CVE-2021-23991 json

If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updat…

published 5y ago modified 4mo ago also EUVD-2021-10911

cvss
6.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.0%
EPSS 1.0% chance of exploitation in 30 days, percentile 64%
public exploits
0
none known

Summary

If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updated key has not yet been imported, an attacker may send an email containing a crafted version of Alice's key with an invalid subkey, Thunderbird might subsequently attempt to use the invalid subkey, and will fail to send encrypted email to Alice. This vulnerability affects Thunderbird < 78.9.1.

Patch / fix

  • Mozilla Thunderbird 78.9.1
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.10.0-1

debian advisories DEBIAN-CVE-2021-23991

  • thunderbird Ubuntu 18.04 LTS 1:78.11.0+build1-0ubuntu0.18.04.2
  • thunderbird Ubuntu 20.04 LTS 1:78.11.0+build1-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2021-23991

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

  • thunderbird RHEL 8 78.9.1-1.el8_3
  • thunderbird RHEL 7 78.9.1-1.el7_9
  • thunderbird RHEL 8.2 EUS 78.9.1-1.el8_2
  • thunderbird RHEL 8.1 EUS 78.9.1-1.el8_1

red hat advisories RHSA-2021:1201RHSA-2021:1193RHSA-2021:1192RHSA-2021:1190

  • MozillaThunderbird SLE Workstation Extension 15 SP2 78.9.1-8.20.1

suse advisories SUSE-SU-2021:1167-1

  • MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
  • MozillaThunderbird openSUSE Leap 15.2 78.9.1-lp152.2.38.1

opensuse advisories openSUSE-SU-2024:10601-1openSUSE-SU-2021:0580-1

  • thunderbird Arch Linux 78.9.1-1

arch linux advisories AVG-1790

Is CVE-2021-23991 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-23991 updated cvss 6.8

Mozilla: An attacker may use Thunderbird's OpenPGP key refresh mechanism to poison an existing key

  • Red Hat Enterprise Linux 6 thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) thunderbird 78.9.1-1.el8_3
    fixed: vendor fix RHSA-2021:1193
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) thunderbird 78.9.1-1.el8_1
    fixed: vendor fix RHSA-2021:1190
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) thunderbird 78.9.1-1.el8_2
    fixed: vendor fix RHSA-2021:1201
  • Red Hat Enterprise Linux Client (v. 7) thunderbird 78.9.1-1.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) thunderbird 78.9.1-1.el7_9 Red Hat Enterprise Linux Workstation (v. 7) thunderbird 78.9.1-1.el7_9
    fixed: vendor fix RHSA-2021:1192

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.04% chance of exploitation in the next 30 days, higher than 64% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • bugzilla.mozilla.org https://bugzilla.mozilla.org/show_bug.cgi?id=1673240 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
NVD 3.1 6.8 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N
NVD 2.0 4.0 AV:N/AC:H/Au:N/C:P/I:P/A:N

Affected products

  • Mozilla Thunderbird

References

Sources

Fencer Intel 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.