CVE-2026-4696: Use-after-free in the Layout: Text and Fonts component

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CRITICAL 9.8REMOTEUPGRADE

CVE-2026-4696 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.

Use-after-free in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Why this priority

Critical severity
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	1.0% (top 38%)
CISA exploited list	No
Weakness	CWE-416, CWE-825
Published	2026-03-24
Last modified	2026-07-15

Publisher (CNA): Mozilla Corporation · Vendor: mozilla

Last updated 2026-07-15 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-24 13:16 UTCPublished CVE record published
2026-07-15 02:22 UTCUpdated Record last modified

mozilla firefox UPGRADE

Affected: < 115.34.0 · < 149.0 · >= 128.0 and < 140.9.0

Fixed in: 115.34.0, 140.9.0, 149.0

Safe to downgrade to: a release before 128.0

1. Check installed version

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

2. UPGRADE to 115.34.0 (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 128.0 (Debian / Ubuntu)

apt-cache madison firefox sudo apt install firefox=<VERSION_BEFORE_128.0> 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_128.0> 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 ELN · perl-Encode · FIXED · 3.25-528.eln159 · FEDORA-2026-79991d0238 · source

sudo dnf upgrade --advisory=FEDORA-2026-79991d0238

Fedora 46 · fonttools · FIXED · 4.68.0-1.fc46 · FEDORA-2026-65d6c01c75 · source

sudo dnf upgrade --advisory=FEDORA-2026-65d6c01c75

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 · 140.9.0esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "140.9.0esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · firefox-esr · FIXED · 140.9.0esr-1~deb13u1

sudo apt update && sudo apt install --only-upgrade firefox-esr dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "140.9.0esr-1~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 12 · thunderbird · FIXED · 1:140.9.0esr-1~deb12u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.9.0esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · thunderbird · FIXED · 1:140.9.0esr-1~deb13u1

sudo apt update && sudo apt install --only-upgrade thunderbird dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:140.9.0esr-1~deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 10 · firefox · FIXED · firefox-140.9.0-1.el10_1

sudo dnf upgrade --advisory=RHSA-2026:5931 rpm -q firefox

RHEL 10 · thunderbird · FIXED · thunderbird-140.9.0-1.el10_1

sudo dnf upgrade --advisory=RHSA-2026:6342 rpm -q thunderbird

RHEL 10.0 Extended Update Support · firefox · FIXED · firefox-140.9.0-1.el10_0

sudo dnf upgrade --advisory=RHSA-2026:7843 rpm -q firefox

RHEL 10.0 Extended Update Support · thunderbird · FIXED · thunderbird-140.9.0-1.el10_0

sudo dnf upgrade --advisory=RHSA-2026:8315 rpm -q thunderbird

RHEL 7 Extended Lifecycle Support · firefox · FIXED · firefox-140.9.0-1.el7_9

sudo dnf upgrade --advisory=RHSA-2026:8427 rpm -q firefox

Amazon Linux 2023 · see advisory · FIXED · ALAS2023-2026-1554 · source

sudo dnf update --advisory=ALAS2023-2026-1554

Amazon Linux 2 · see advisory · FIXED · ALAS2-2026-3241 · source

sudo yum update --advisory=ALAS2-2026-3241

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-2026-4696?

firefox: upgrade to 115.34.0, 140.9.0, 149.0. Exact commands for each OS are on this page.

Is CVE-2026-4696 being exploited?

CVE-2026-4696 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-2026-4696 affect?

CVE-2026-4696 affects firefox. Severity is critical (CVSS 9.8). Use-after-free in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

How urgent is CVE-2026-4696?

Priority 58/100. Critical severity; CISA SSVC: automatable with total technical impact; Remotely exploitable, no login, no user action

Software pages

Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities

Related CVEs

CVE-2026-100801 HIGH 8.8 UPGRADE UPGRADE Privilege escalation in the DLL Services component
CVE-2026-6746 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the DOM: Core & HTML component
CVE-2026-6758 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the JavaScript: WebAssembly component
CVE-2026-6771 CRITICAL 9.8 UPGRADE UPGRADE Mitigation bypass in the DOM: Security component
CVE-2026-6747 HIGH 7.5 UPGRADE UPGRADE Use-after-free in the WebRTC component

References

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