CVE-2026-5731: Memory safety bugs present in Firefox ESR 115.34.0, Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.

This month

CRITICAL 9.8REMOTEDOWNGRADEDOWNGRADE

CVE-2026-5731 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 115.34.0, 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 115.34.1, 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 51%)
CISA exploited list	No
Weakness	CWE-119, CWE-787
Published	2026-04-07
Last modified	2026-10-24

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-10-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-07 13:16 UTCPublished CVE record published
2026-10-24 13:10 UTCUpdated Record last modified

mozilla firefox DOWNGRADE

Affected: 115.34.0 · 140.9.0 · 149.0.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 115.34.0

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 115.34.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_115.34.0> sudo apt-mark hold firefox

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list firefox sudo dnf downgrade firefox-<VERSION_BEFORE_115.34.0> sudo dnf versionlock add firefox

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

mozilla thunderbird DOWNGRADE

Affected: 140.9.0 · 149.0.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 140.9.0

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 140.9.0 (Debian / Ubuntu)

apt-cache madison thunderbird sudo apt install thunderbird=<VERSION_BEFORE_140.9.0> sudo apt-mark hold thunderbird

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list thunderbird sudo dnf downgrade thunderbird-<VERSION_BEFORE_140.9.0> sudo dnf versionlock add thunderbird

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · gnome-shell-extension-tiling-assistant · FIXED · 56-1.fc46 · FEDORA-2026-ebf66a5321 · source

sudo dnf upgrade --advisory=FEDORA-2026-ebf66a5321

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.

Quick answers

How do I fix CVE-2026-5731?

firefox: downgrade or apply the listed mitigation; thunderbird: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-5731 being exploited?

CVE-2026-5731 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-5731 affect?

CVE-2026-5731 affects firefox, thunderbird. Severity is critical (CVSS 9.8). Memory safety bugs present in Firefox ESR 115.34.0, 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 o

How urgent is CVE-2026-5731?

Priority 51/100. Critical severity; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities

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References

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