Out-of-bounds read in Firefox ESR - CVE-2026-8959

Vulnerability identifier: #VU131844

CSH Severity: Low

CVSS v4: 8.5 [CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-8959

CWE-ID: CWE-125

Exploitation vector: Local access

Exploit availability: No public exploit available

The vulnerability allows a local user to escape the sandbox.

The vulnerability exists due to incorrect boundary conditions in the Widget: Win32 component when handling local interaction with the application. A local user can trigger the flaw to escape the sandbox.

The issue affects the Win32-specific component.

Firefox ESR
Mozilla Firefox
SUSE Linux Enterprise Server 15 SP6
SUSE Linux Enterprise Server 12 SP5
SUSE Linux Enterprise Server 12 SP5 LTSS Extended
SUSE Linux Enterprise Server 15 SP5
SUSE Linux Enterprise Server 15 SP4
SUSE Linux Enterprise Server for SAP Applications 15
SUSE Linux Enterprise High Performance Computing 15
SUSE Linux Enterprise High Performance Computing ESPOS 15
SUSE Linux Enterprise High Performance Computing LTSS 15
SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server for SAP Applications 12
SUSE Linux Enterprise Server 12
SUSE Linux Enterprise High Performance Computing 12
SUSE Linux Enterprise Desktop 15
SUSE Linux Enterprise Real Time 15
SUSE Linux Enterprise Workstation Extension 15
Anolis OS
Red Hat Enterprise Linux for x86_64
Red Hat Enterprise Linux for ARM 64
Red Hat Enterprise Linux for IBM z Systems
Red Hat Enterprise Linux for Power, little endian
Red Hat Enterprise Linux Server - AUS
Red Hat Enterprise Linux Server - TUS
Red Hat Enterprise Linux for x86_64 - Extended Update Support
Red Hat Enterprise Linux for IBM z Systems - Extended Update Support
Red Hat Enterprise Linux for Power, little endian - Extended Update Support
Red Hat Enterprise Linux for ARM 64 - Extended Update Support
SUSE Package Hub 15
Desktop Applications Module
openEuler
Red Hat Enterprise Linux Server for Power LE - Update Services for SAP Solutions
firefox-debuginfo
firefox-debugsource
firefox
thunderbird (Red Hat package)
MozillaFirefox-debugsource
MozillaFirefox-debuginfo
MozillaFirefox-translations-common
MozillaFirefox-devel
MozillaFirefox
MozillaThunderbird-translations-other
MozillaThunderbird-debugsource
MozillaThunderbird
MozillaThunderbird-debuginfo
MozillaThunderbird-translations-common
MozillaFirefox-translations-other
thunderbird-librnp-rnp
thunderbird-wayland
thunderbird
thunderbird-debuginfo
thunderbird-debugsource
Mozilla Thunderbird
Firefox for Android

Install security update from vendor's website.

Firefox ESR - update to 140.11.0
Mozilla Thunderbird - addressed in versions 140.11.0, 151.0
Mozilla Firefox - update to 151.0
Firefox for Android - update to 151.0
firefox-debuginfo - update to 140.11.0-1
firefox-debugsource - update to 140.11.0-1
firefox - update to 140.11.0-1
thunderbird (Red Hat package) - addressed in versions 140.11.0-1.el8_4, 140.11.0-1.el8_6, 140.11.0-1.el8_8, 140.11.0-1.el8_10, 140.11.0-1.el9_2, 140.11.0-1.el9_4, 140.11.0-1.el9_6, 140.11.0-1.el9_8, 140.11.0-1.el10_0, 140.11.0-1.el10_2
MozillaFirefox-debugsource - addressed in versions 140.11.0-112.318.1, 140.11.0-150200.152.239.1
MozillaFirefox-debuginfo - addressed in versions 140.11.0-112.318.1, 140.11.0-150200.152.239.1
MozillaFirefox-translations-common - addressed in versions 140.11.0-112.318.1, 140.11.0-150200.152.239.1
MozillaFirefox-devel - addressed in versions 140.11.0-112.318.1, 140.11.0-150200.152.239.1
MozillaFirefox - addressed in versions 140.11.0-112.318.1, 140.11.0-150200.152.239.1
MozillaThunderbird-translations-other - update to 140.11.0-150200.8.274.1
MozillaThunderbird-debugsource - update to 140.11.0-150200.8.274.1
MozillaThunderbird - update to 140.11.0-150200.8.274.1
MozillaThunderbird-debuginfo - update to 140.11.0-150200.8.274.1
MozillaThunderbird-translations-common - update to 140.11.0-150200.8.274.1
MozillaFirefox-translations-other - update to 140.11.0-150200.152.239.1
thunderbird-librnp-rnp - update to 140.12.0-1
thunderbird-wayland - update to 140.12.0-1
thunderbird - update to 140.12.0-1
thunderbird-debuginfo - update to 140.12.0-1
thunderbird-debugsource - update to 140.12.0-1
thunderbird - update to 140.12.0-1.0.1

• https://www.mozilla.org/en-US/security/advisories/mfsa2026-48/
• https://bugzilla.mozilla.org/show_bug.cgi?id=2034754

• Multiple vulnerabilities in Mozilla Firefox ESR 140
• Multiple vulnerabilities in Mozilla Firefox
• SUSE update for MozillaFirefox
• Multiple vulnerabilities in Thunderbird ESR
• Multiple vulnerabilities in Mozilla Thunderbird
• openEuler 24.03 LTS update for firefox
• openEuler 22.03 LTS SP4 update for firefox
• openEuler 24.03 LTS SP3 update for firefox
• Red Hat Enterprise Linux 9 update for thunderbird
• SUSE update for MozillaFirefox
• openEuler 24.03 LTS SP1 update for firefox
• Red Hat Enterprise Linux 10 update for thunderbird
• Red Hat Enterprise Linux 8 update for thunderbird
• SUSE update for MozillaThunderbird
• Red Hat Enterprise Linux 9 update for thunderbird
• Red Hat Enterprise Linux 8 update for thunderbird
• Red Hat Enterprise Linux 9 update for thunderbird
• Red Hat Enterprise Linux 8 update for thunderbird
• Red Hat Enterprise Linux 9 update for thunderbird
• Red Hat Enterprise Linux 8 update for thunderbird
• Red Hat Enterprise Linux 10 update for thunderbird
• openEuler 24.03 LTS SP3 update for thunderbird
• Anolis OS update for thunderbird