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Fencer Intel
critical 9.1

CVE-2026-84639 json

Uninitialized memory in MIME parsing

published 2mo ago modified last mo. also EUVD-2026-69708

cvss
9.1
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 25%
public exploits
0
none known

Summary

Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Patch / fix

  • mozilla thunderbird 140.15.0, 153.2.0, 155.0
  • thunderbird Debian 12 1:140.15.0esr-1~deb12u1
  • thunderbird Debian 13 1:140.15.0esr-1~deb13u1
  • thunderbird Debian 14 1:153.2.0esr-1

debian advisories DEBIAN-CVE-2026-84639

  • thunderbird RHEL 8 140.16.0-1.el8_10
  • thunderbird RHEL 10.2 140.16.0-1.el10_2
  • thunderbird RHEL 9 140.16.0-1.el9_8
  • thunderbird RHEL 8.4 EUS_LONG_LIFE, RHEL 8.4 AUS 140.16.0-1.el8_4
  • thunderbird RHEL 8.6 EUS_LONG_LIFE, RHEL 8.6 AUS 140.16.0-1.el8_6
  • thunderbird RHEL 8.8 TUS, RHEL 8.8 E4S 140.16.0-1.el8_8
  • thunderbird RHEL 9.6 EUS 140.15.0-1.el9_6
  • thunderbird RHEL 9.2 E4S 140.15.0-1.el9_2

red hat advisories RHSA-2026:82122RHSA-2026:81913RHSA-2026:81912RHSA-2026:80503RHSA-2026:77632RHSA-2026:77631

  • MozillaThunderbird SLE Workstation Extension 15 SP7, SLE Module for Package Hub 15 SP7 153.2.0-150400.13.3.1

suse advisories SUSE-SU-2026:4268-1

  • MozillaThunderbird openSUSE Tumbleweed 140.15.0-1.1

opensuse advisories openSUSE-SU-2026:11678-1

  • thunderbird AlmaLinux 8 140.16.0-1.el8_10.alma.1
  • thunderbird AlmaLinux 10 140.16.0-1.el10_2.alma.1
  • thunderbird AlmaLinux 9 140.16.0-1.el9_8.alma.1

almalinux advisories ALSA-2026:73971ALSA-2026:73130ALSA-2026:70642

  • thunderbird Rocky Linux 8 140.16.0-1.el8_10
  • thunderbird Rocky Linux 10 140.16.0-1.el10_2
  • thunderbird Rocky Linux 9 140.16.0-1.el9_8

rocky linux advisories RLSA-2026:73971RLSA-2026:73130RLSA-2026:70642

Is CVE-2026-84639 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.

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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-2026-84639 updated cvss 7.5

thunderbird: Uninitialized memory in MIME parsing

  • Red Hat Enterprise Linux 6 thunderbird Red Hat Enterprise Linux 7 thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 10 rhel10/thunderbird-flatpak
    affected: no fix available
  • Red Hat Enterprise Linux AppStream (v. 8) thunderbird 140.16.0-1.el8_10
    fixed: vendor fix RHSA-2026:73971
  • Red Hat Enterprise Linux AppStream (v. 9) thunderbird 140.16.0-1.el9_8
    fixed: vendor fix RHSA-2026:70642
  • Red Hat Enterprise Linux AppStream (v. 10) thunderbird 140.16.0-1.el10_2
    fixed: vendor fix RHSA-2026:73130
  • Red Hat Enterprise Linux AppStream AUS (v.8.4) thunderbird 140.16.0-1.el8_4
    fixed: vendor fix RHSA-2026:82122
  • Red Hat Enterprise Linux AppStream AUS (v.8.6) thunderbird 140.16.0-1.el8_6
    fixed: vendor fix RHSA-2026:81913
  • Red Hat Enterprise Linux AppStream E4S (v.8.8) thunderbird 140.16.0-1.el8_8
    fixed: vendor fix RHSA-2026:81912
show 7 more
  • Red Hat Enterprise Linux AppStream E4S (v.9.2) thunderbird 140.15.0-1.el9_2
    fixed: vendor fix RHSA-2026:77632
  • Red Hat Enterprise Linux AppStream E4S (v.9.4) thunderbird 140.15.0-1.el9_4
    fixed: vendor fix RHSA-2026:77627
  • Red Hat Enterprise Linux AppStream EUS (v. 10.0) thunderbird 140.15.0-1.el10_0
    fixed: vendor fix RHSA-2026:77631
  • Red Hat Enterprise Linux AppStream EUS (v.9.6) thunderbird 140.15.0-1.el9_6
    fixed: vendor fix RHSA-2026:80503
  • Red Hat Enterprise Linux AppStream EUS EXTENSION (v.8.4) thunderbird 140.16.0-1.el8_4
    fixed: vendor fix RHSA-2026:82122
  • Red Hat Enterprise Linux AppStream EUS EXTENSION (v.8.6) thunderbird 140.16.0-1.el8_6
    fixed: vendor fix RHSA-2026:81913
  • Red Hat Enterprise Linux AppStream TUS (v.8.8) thunderbird 140.16.0-1.el8_8
    fixed: vendor fix RHSA-2026:81912

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

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
2mo ago

Exploit prediction

0.32% chance of exploitation in the next 30 days, higher than 25% of all scored CVEs (FIRST EPSS, 2026-11-11).

CVSS scores

assigned by version score vector
CISA ADP 3.1 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
CNA 3.1 9.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H

Affected products

  • Mozilla Thunderbird

Classification

Weakness
  • CWE-457 Use of Uninitialized Variable

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.