CVE-2025-6424: A use-after-free in FontFaceSet resulted in a potentially exploitable crash

This week

CRITICAL 9.8REMOTEUPGRADE

CVE-2025-6424 is a critical-severity flaw (CVSS 9.8) affecting firefox. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A use-after-free in FontFaceSet resulted in a potentially exploitable crash. This vulnerability was fixed in Firefox 140, Firefox ESR 115.25, Firefox ESR 128.12, Thunderbird 140, and Thunderbird 128.12.

Why this priority

Critical severity
EPSS top 9%: very likely to be exploited
CISA SSVC: automatable with total technical impact
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	4.4% (top 9%)
CISA exploited list	No
Weakness	CWE-416
Published	2025-06-24
Last modified	2026-10-23

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Am I affected?

Timeline

2025-06-24 13:15 UTCPublished CVE record published
2026-10-23 22:10 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 115.25.0 · < 140.0 · >= 116.0 and < 128.12.0

Fixed in: 115.25.0, 128.12.0, 140.0

Safe to downgrade to: a release before 116.0

1. Check installed version

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

2. UPGRADE to 115.25.0 (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.

3. If no update is available yet, DOWNGRADE to a release before 116.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_116.0> sudo apt-mark hold firefox

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list firefox sudo dnf downgrade firefox-<VERSION_BEFORE_116.0> 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.

Exact fixes by distribution

Fedora 46 · pspp · FIXED · 2.1.3-2.fc46 · FEDORA-2026-0d6444911b · source

sudo dnf upgrade --advisory=FEDORA-2026-0d6444911b

Fedora ELN · barman · FIXED · 3.20.1-3.eln159 · FEDORA-2026-45ecf042c2 · source

sudo dnf upgrade --advisory=FEDORA-2026-45ecf042c2

Ubuntu 22.04 · firefox · NOT AFFECTED

Ubuntu 24.04 · firefox · NOT AFFECTED

Ubuntu 24.10 · firefox · NOT AFFECTED

Ubuntu 25.04 · firefox · NOT AFFECTED

Ubuntu 25.10 · firefox · NOT AFFECTED

Debian 12 · firefox-esr · FIXED · 128.12.0esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "128.12.0esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · firefox-esr · FIXED · 128.12.0esr-1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "128.12.0esr-1" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · thunderbird · FIXED · 1:128.12.0esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:128.12.0esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · thunderbird · FIXED · 1:128.12.0esr-1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:128.12.0esr-1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · firefox · FIXED · firefox-128.12.0-1.el10_0

sudo dnf upgrade --advisory=RHSA-2025:10073 rpm -q firefox

RHEL 10 · thunderbird · FIXED · thunderbird-128.12.0-1.el10_0

sudo dnf upgrade --advisory=RHSA-2025:10195 rpm -q thunderbird

RHEL 7 Extended Lifecycle Support · firefox · FIXED · firefox-128.12.0-1.el7_9

sudo dnf upgrade --advisory=RHSA-2025:10181 rpm -q firefox

RHEL 8 · firefox · FIXED · firefox-128.12.0-1.el8_10

sudo dnf upgrade --advisory=RHSA-2025:10074 rpm -q firefox

RHEL 8 · thunderbird · FIXED · thunderbird-128.12.0-1.el8_10

sudo dnf upgrade --advisory=RHSA-2025:10246 rpm -q thunderbird

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2025-1055 · source

sudo dnf update --advisory=ALAS2023-2025-1055

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

Write-up: bugzilla.mozilla.org/show_bug.cgi?id=1966423

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.

Related CVEs

CVE-2023-4050 HIGH 7.5 UPGRADE MITIGATE In some cases, an untrusted input stream was copied to a stack buffer without checking its size
CVE-2023-29542 CRITICAL 9.8 UPGRADE UPGRADE A newline in a filename could have been used to bypass the file extension security mechanisms that replace mal
CVE-2025-11708 CRITICAL 9.8 UPGRADE UPGRADE Use-after-free in MediaTrackGraphImpl::GetInstance()
CVE-2026-5734 CRITICAL 9.8 UPGRADE UPGRADE Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 14
CVE-2026-5735 CRITICAL 9.8 UPGRADE UPGRADE Memory safety bugs present in Firefox 149.0.1 and Thunderbird 149.0.1

References

← Back to latest CVEs