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Fencer Vulns
medium 6.5

CVE-2021-23993 json

An attacker may perform a DoS attack to prevent a user from sending encrypted email to a correspondent.

published 2021-06-24 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-10913

cvss
6.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 36%
public exploits
0
none known

Summary

An attacker may perform a DoS attack to prevent a user from sending encrypted email to a correspondent. If an attacker creates a crafted OpenPGP key with a subkey that has an invalid self signature, and the Thunderbird user imports the crafted key, then Thunderbird may try to use the invalid subkey, but the RNP library rejects it from being used, causing encryption to fail. 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-2021-23993

  • 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-2021-23993

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 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

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-2021-23993 updated cvss 6.5

Mozilla: Inability to send encrypted OpenPGP email after importing a crafted OpenPGP 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

0.42% chance of exploitation in the next 30 days, higher than 36% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • Mozilla Thunderbird

Classification

Weakness
  • CWE-347 Improper Verification of Cryptographic Signature

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.