PatchTheCVECVE-2025-4918: An attacker was able to perform an out-of-bounds read or write on a JavaScript `Promise` object
68
This week
CRITICAL 9.8REMOTEUPGRADEUPGRADE
CVE-2025-4918 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.
An attacker was able to perform an out-of-bounds read or write on a JavaScript `Promise` object. This vulnerability was fixed in Firefox 138.0.4, Firefox ESR 128.10.1, Firefox ESR 115.23.1, Thunderbird 128.10.2, and Thunderbird 138.0.2.
Why this priority
- Critical severity
- EPSS top 5%: very likely to be exploited
- Public exploit reference
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Publisher (CNA): Mozilla Corporation · Vendor: mozilla
Last updated 2026-09-30 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
mozilla firefox UPGRADE
Affected: < 115.23.1 · < 138.0.4 · >= 116.0 and < 128.10.1
Fixed in: 115.23.1, 128.10.1, 138.0.4
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 115.23.1 (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.
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.
mozilla thunderbird UPGRADE
Affected: < 128.10.2 · >= 138.0 and < 138.0.2
Fixed in: 128.10.2, 138.0.2
Safe to downgrade to: a release before 138.0
1. Check installed version
apt-cache policy thunderbird || rpm -q thunderbird || thunderbird --version
2. UPGRADE to 128.10.2 (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.
3. If no update is available yet, DOWNGRADE to a release before 138.0 (Debian / Ubuntu)
apt-cache madison thunderbird
sudo apt install thunderbird=<VERSION_BEFORE_138.0>
sudo apt-mark hold thunderbird
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list thunderbird
sudo dnf downgrade thunderbird-<VERSION_BEFORE_138.0>
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
Fedora ELN · fips-provider-next · FIXED · 1.5.3-3.eln159 · FEDORA-2026-3ce99ec7df · source
sudo dnf upgrade --advisory=FEDORA-2026-3ce99ec7df
Fedora 46 · rubygem-webmock · FIXED · 3.26.1-3.fc46 · FEDORA-2026-bb57b4d095 · source
sudo dnf upgrade --advisory=FEDORA-2026-bb57b4d095
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 · 128.10.1esr-1~deb12u1
sudo apt update && sudo apt install --only-upgrade firefox-esr
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "128.10.1esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE
Debian 13 · firefox-esr · FIXED · 128.10.1esr-1
sudo apt update && sudo apt install --only-upgrade firefox-esr
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' firefox-esr)" ge "128.10.1esr-1" && echo PATCHED || echo STILL-VULNERABLE
Debian 12 · thunderbird · FIXED · 1:128.11.0esr-1~deb12u1
sudo apt update && sudo apt install --only-upgrade thunderbird
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:128.11.0esr-1~deb12u1" && echo PATCHED || echo STILL-VULNERABLE
Debian 13 · thunderbird · FIXED · 1:128.11.0esr-1
sudo apt update && sudo apt install --only-upgrade thunderbird
dpkg --compare-versions "$(dpkg-query -W -f='${Version}' thunderbird)" ge "1:128.11.0esr-1" && echo PATCHED || echo STILL-VULNERABLE
RHEL 10 · firefox · FIXED · firefox-128.10.1-1.el10_0
sudo dnf upgrade --advisory=RHSA-2025:8125
rpm -q firefox
RHEL 10 · thunderbird · FIXED · thunderbird-128.11.0-1.el10_0
sudo dnf upgrade --advisory=RHSA-2025:8608
rpm -q thunderbird
RHEL 7 Extended Lifecycle Support · firefox · FIXED · firefox-128.10.1-1.el7_9
sudo dnf upgrade --advisory=RHSA-2025:8465
rpm -q firefox
RHEL 8 · firefox · FIXED · firefox-128.10.1-1.el8_10
sudo dnf upgrade --advisory=RHSA-2025:8060
rpm -q firefox
RHEL 8 · thunderbird · FIXED · thunderbird-128.11.0-1.el8_10
sudo dnf upgrade --advisory=RHSA-2025:8756
rpm -q thunderbird
sudo dnf update --advisory=ALAS2023-2025-996
sudo yum update --advisory=ALAS2-2025-2873
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=1966612
- Write-up: www.vicarius.io/vsociety/posts/cve-2025-4918-detect-firefox-out-of-bounds-write
- Write-up: www.vicarius.io/vsociety/posts/cve-2025-4918-mitigate-firefox-out-of-bounds-write
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.
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-2025-4918?
firefox: upgrade to 115.23.1, 128.10.1, 138.0.4; thunderbird: upgrade to 128.10.2, 138.0.2. Exact commands for each OS are on this page.
Is CVE-2025-4918 being exploited?
CVE-2025-4918 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-2025-4918 affect?
CVE-2025-4918 affects firefox, thunderbird. Severity is critical (CVSS 9.8). An attacker was able to perform an out-of-bounds read or write on a JavaScript `Promise` object. This vulnerability was fixed in Firefox 138.0.4, Firefox ESR 128.10.1, Firefox ESR 115.23.1, Thunderbird 128.10.2, and Thunderbird 138.0.2.
How urgent is CVE-2025-4918?
Priority 68/100. Critical severity; EPSS top 5%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action
Software pages
Mozilla Firefox vulnerabilities · Mozilla Thunderbird vulnerabilities
Related CVEs
- CVE-2026-74990 CRITICAL 9.8 UPGRADE UPGRADE Internally found bugs present in Firefox ESR 115.38, Firefox ESR 140.13, Firefox ESR 153.0 and Firefox 153
- CVE-2026-74989 CRITICAL 9.8 UPGRADE UPGRADE Internally found bugs present in Firefox 153
- CVE-2026-74988 CRITICAL 9.8 UPGRADE UPGRADE Internally found bugs present in Firefox ESR 153.0 and Firefox 153
- CVE-2026-74987 CRITICAL 9.8 UPGRADE UPGRADE Internally found bugs present in Firefox ESR 140.13, Firefox ESR 153.0 and Firefox 153
- CVE-2026-74969 HIGH 8.8 UPGRADE UPGRADE Race condition, use-after-free in the Layout: Text and Fonts component