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Fencer Vulns
medium 5.9

CVE-2021-29969 json

If Thunderbird was configured to use STARTTLS for an IMAP connection, and an attacker injected IMAP server responses prior to the completio…

published 2021-08-05 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-16428

cvss
5.9
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
1.2%
EPSS 1.2% chance of exploitation in 30 days, percentile 68%
public exploits
0
none known

Summary

If Thunderbird was configured to use STARTTLS for an IMAP connection, and an attacker injected IMAP server responses prior to the completion of the STARTTLS handshake, then Thunderbird didn't ignore the injected data. This could have resulted in Thunderbird showing incorrect information, for example the attacker could have tricked Thunderbird to show folders that didn't exist on the IMAP server. This vulnerability affects Thunderbird < 78.12.

Patch / fix

  • Mozilla Thunderbird 78.12
  • thunderbird Debian 14, Debian 13, Debian 12 1:78.12.0-1

debian advisories DEBIAN-CVE-2021-29969

  • thunderbird Ubuntu 18.04 LTS 1:78.13.0+build1-0ubuntu0.18.04.1
  • thunderbird Ubuntu 20.04 LTS 1:78.13.0+build1-0ubuntu0.20.04.2

ubuntu advisories UBUNTU-CVE-2021-29969

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 7 78.12.0-2.el7_9
  • thunderbird RHEL 8 78.12.0-3.el8_4
  • thunderbird RHEL 8.2 EUS 78.12.0-2.el8_2
  • thunderbird RHEL 8.1 EUS 78.12.0-2.el8_1

red hat advisories RHSA-2021:2914RHSA-2021:2881RHSA-2021:2882RHSA-2021:2883

  • MozillaThunderbird SLE Workstation Extension 15 SP3, SLE Workstation Extension 15 SP2 78.12.0-8.33.1

suse advisories SUSE-SU-2021:2458-1

  • MozillaThunderbird openSUSE Tumbleweed 91.1.1-1.1
  • MozillaThunderbird openSUSE Leap 15.2 78.12.0-lp152.2.48.2
  • MozillaThunderbird openSUSE Leap 15.3 78.12.0-8.33.1

opensuse advisories openSUSE-SU-2024:10601-1openSUSE-SU-2021:1091-1openSUSE-SU-2021:2458-1

  • thunderbird Rocky Linux 8 78.12.0-3.el8_4

rocky linux advisories RLSA-2021:2883

  • thunderbird Alpine 3.23, Alpine 3.22, Alpine 3.21 +5 more 91.3.2-r0

alpine advisories ALPINE-CVE-2021-29969

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

  • thunderbird Arch Linux 78.12.0-1

arch linux advisories AVG-2152

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-2021-29969 updated cvss 5.9

Mozilla: IMAP server responses sent by a MITM prior to STARTTLS could be processed

  • Red Hat Enterprise Linux 6 thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux AppStream (v. 8) thunderbird 78.12.0-3.el8_4
    fixed: vendor fix RHSA-2021:2883
  • Red Hat Enterprise Linux AppStream EUS (v. 8.1) thunderbird 78.12.0-2.el8_1
    fixed: vendor fix RHSA-2021:2882
  • Red Hat Enterprise Linux AppStream EUS (v. 8.2) thunderbird 78.12.0-2.el8_2
    fixed: vendor fix RHSA-2021:2914
  • Red Hat Enterprise Linux Client (v. 7) thunderbird 78.12.0-2.el7_9 Red Hat Enterprise Linux Server Optional (v. 7) thunderbird 78.12.0-2.el7_9 Red Hat Enterprise Linux Workstation (v. 7) thunderbird 78.12.0-2.el7_9
    fixed: vendor fix RHSA-2021:2881

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

1.20% chance of exploitation in the next 30 days, higher than 68% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 5.9 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:P/I:N/A:N

Affected products

  • Mozilla Thunderbird

Classification

Weakness
  • CWE-552 Files or Directories Accessible to External Parties

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.