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Fencer Intel
critical 9.6

CVE-2026-8959 json

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component

published 5mo ago modified 28d ago also EUVD-2026-30910

cvss
9.6
version 3.1, CNA
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

Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component. This vulnerability was fixed in Firefox 151, Firefox ESR 140.11, Thunderbird 151, and Thunderbird 140.11.

Patch / fix

  • mozilla firefox 140.11, 151
  • mozilla firefox_esr 140.11, 151
  • mozilla thunderbird 140.11, 151
  • mozilla thunderbird_esr 140.11, 151
  • thunderbird RHEL 8 140.11.0-1.el8_10
  • thunderbird RHEL 10.2 140.11.0-1.el10_2
  • thunderbird RHEL 9 140.11.0-1.el9_8
  • thunderbird enterprise linux eus 10.0 140.11.0-1.el10_0
  • thunderbird RHEL 8.6 EUS_LONG_LIFE, RHEL 8.6 AUS 140.11.0-1.el8_6
  • thunderbird RHEL 9.6 EUS 140.11.0-1.el9_6
  • thunderbird RHEL 8.8 TUS, RHEL 8.8 E4S 140.11.0-1.el8_8
  • thunderbird RHEL 9.2 E4S 140.11.0-1.el9_2

red hat advisories RHSA-2026:26539RHSA-2026:26536RHSA-2026:26521RHSA-2026:26268RHSA-2026:26269RHSA-2026:26270

  • MozillaThunderbird openSUSE Leap 16.0 140.12.0-bp160.1.1
  • MozillaFirefox openSUSE Tumbleweed 151.0.1-1.1
  • MozillaThunderbird openSUSE Tumbleweed 140.11.1-1.1
  • firefox-esr openSUSE Tumbleweed 140.11.0-1.1

opensuse advisories openSUSE-SU-2026:21168-1openSUSE-SU-2026:10863-1openSUSE-SU-2026:10864-1openSUSE-SU-2026:10813-1

  • thunderbird AlmaLinux 8 140.11.0-1.el8_10.alma.1
  • thunderbird AlmaLinux 10 140.11.0-1.el10_2.alma.1
  • thunderbird AlmaLinux 9 140.11.0-1.el9_8.alma.1

almalinux advisories ALSA-2026:22643ALSA-2026:22325ALSA-2026:21381

  • thunderbird Rocky Linux 8 140.11.0-1.el8_10
  • thunderbird Rocky Linux 10 140.11.0-1.el10_2
  • thunderbird Rocky Linux 9 140.11.0-1.el9_8

rocky linux advisories RLSA-2026:22643RLSA-2026:22325RLSA-2026:21381

Is CVE-2026-8959 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-2026-8959 updated cvss 6.1

firefox: thunderbird: Sandbox escape due to incorrect boundary conditions in the Widget: Win32 component

  • 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 140.11.0-1.el8_10
    fixed: vendor fix RHSA-2026:22643
  • Red Hat Enterprise Linux AppStream (v. 9) thunderbird 140.11.0-1.el9_8
    fixed: vendor fix RHSA-2026:21381
  • Red Hat Enterprise Linux AppStream (v. 10) thunderbird 140.11.0-1.el10_2
    fixed: vendor fix RHSA-2026:22325
  • Red Hat Enterprise Linux AppStream AUS (v.8.4) thunderbird 140.11.0-1.el8_4
    fixed: vendor fix RHSA-2026:26270
  • Red Hat Enterprise Linux AppStream AUS (v.8.6) thunderbird 140.11.0-1.el8_6
    fixed: vendor fix RHSA-2026:26536
  • Red Hat Enterprise Linux AppStream E4S (v.8.8) thunderbird 140.11.0-1.el8_8
    fixed: vendor fix RHSA-2026:26268
show 8 more
  • Red Hat Enterprise Linux AppStream E4S (v.9.2) thunderbird 140.11.0-1.el9_2
    fixed: vendor fix RHSA-2026:26269
  • Red Hat Enterprise Linux AppStream E4S (v.9.4) thunderbird 140.11.0-1.el9_4
    fixed: vendor fix RHSA-2026:26174
  • Red Hat Enterprise Linux AppStream EUS (v. 10.0) thunderbird 140.11.0-1.el10_0
    fixed: vendor fix RHSA-2026:26539
  • Red Hat Enterprise Linux AppStream EUS (v.9.6) thunderbird 140.11.0-1.el9_6
    fixed: vendor fix RHSA-2026:26521
  • Red Hat Enterprise Linux AppStream EUS EXTENSION (v.8.4) thunderbird 140.11.0-1.el8_4
    fixed: vendor fix RHSA-2026:26270
  • Red Hat Enterprise Linux AppStream EUS EXTENSION (v.8.6) thunderbird 140.11.0-1.el8_6
    fixed: vendor fix RHSA-2026:26536
  • Red Hat Enterprise Linux AppStream TUS (v.8.8) thunderbird 140.11.0-1.el8_8
    fixed: vendor fix RHSA-2026:26268
  • Red Hat Enterprise Linux 6 firefox Red Hat Enterprise Linux 7 firefox Red Hat Enterprise Linux 8 firefox Red Hat Enterprise Linux 9 firefox, firefox-x11 Red Hat Enterprise Linux 10 firefox, rhel10/firefox-flatpak
    not affected: vulnerable code not present

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
no
Technical impact
total
Assessed
5mo ago

Exploit prediction

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

CVSS scores

assigned by version score vector
CISA ADP 3.1 9.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
CNA 3.1 9.6 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Affected products

  • Mozilla Firefox
  • Mozilla Thunderbird

Classification

Weakness
  • CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer
  • CWE-20 Improper Input Validation
  • CWE-693 Protection Mechanism Failure

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.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.