CVE-2026-5734: Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 14

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CRITICAL 9.8REMOTEUPGRADEUPGRADE

CVE-2026-5734 is a critical-severity flaw (CVSS 9.8) affecting firefox, thunderbird. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

Why this priority

Critical severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 52%)
CISA exploited list	No
Weakness	CWE-787, CWE-120
Published	2026-04-07
Last modified	2026-10-23

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Am I affected?

Timeline

2026-04-07 13:16 UTCPublished CVE record published
2026-10-23 23:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 140.9.1 · < 149.0.2

Fixed in: 140.9.1, 149.0.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy firefox || rpm -q firefox || firefox --version

2. UPGRADE to 140.9.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade firefox

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade firefox

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

mozilla thunderbird UPGRADE

Affected: < 140.9.1 · < 149.0.2

Fixed in: 140.9.1, 149.0.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version

2. UPGRADE to 140.9.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade thunderbird

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade thunderbird

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Exact fixes by distribution

Fedora 46 · python-openslide · FIXED · 1.4.7-1.fc46 · FEDORA-2026-336bea884e · source

sudo dnf upgrade --advisory=FEDORA-2026-336bea884e

Fedora ELN · valgrind · FIXED · 3.28.0-1.eln159 · FEDORA-2026-c593ef1d15 · source

sudo dnf upgrade --advisory=FEDORA-2026-c593ef1d15

Ubuntu 22.04 · firefox · NOT AFFECTED

Ubuntu 24.04 · firefox · NOT AFFECTED

Ubuntu 25.10 · firefox · NOT AFFECTED

Ubuntu 22.04 · thunderbird · NO FIX YET

Ubuntu 24.04 · thunderbird · NOT AFFECTED

Debian 12 · firefox-esr · FIXED · 140.9.1esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "140.9.1esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · firefox-esr · FIXED · 140.9.1esr-1~deb13u1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "140.9.1esr-1~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · thunderbird · FIXED · 1:140.9.1esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.9.1esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · thunderbird · FIXED · 1:140.9.1esr-1~deb13u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.9.1esr-1~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · firefox · FIXED · firefox-140.9.1-1.el10_1

sudo dnf upgrade --advisory=RHSA-2026:7672 rpm -q firefox

RHEL 10 · thunderbird · FIXED · thunderbird-140.9.1-1.el10_1

sudo dnf upgrade --advisory=RHSA-2026:9638 rpm -q thunderbird

RHEL 10.0 Extended Update Support · thunderbird · FIXED · thunderbird-140.9.1-1.el10_0

sudo dnf upgrade --advisory=RHSA-2026:11813 rpm -q thunderbird

RHEL 10.0 Extended Update Support · firefox · FIXED · firefox-140.9.1-1.el10_0

sudo dnf upgrade --advisory=RHSA-2026:13665 rpm -q firefox

RHEL 7 Extended Lifecycle Support · firefox · FIXED · firefox-140.9.1-2.el7_9

sudo dnf upgrade --advisory=RHSA-2026:13977 rpm -q firefox

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-1621 · source

sudo dnf update --advisory=ALAS2023-2026-1621

Amazon Linux 2 · see advisory · FIXED · ALAS2-2026-3271 · source

sudo yum update --advisory=ALAS2-2026-3271

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

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References

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