⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 8.8

How to Fix CVE-2026-2799: Use-after-free in Firefox

By Sai Kiran Pandrala. Last verified: 2026-05-25.

CVE-2026-2799 is a use-after-free in Mozilla Firefox. The fix is to upgrade to Firefox 148; Thunderbird 148.

⚡ At a glance
Severity8.8 (High)
Actively exploited?No public listing in CISA KEV
AffectedSee vendor advisory
Fixed inFirefox 148; Thunderbird 148
Type (CWE)CWE-416 Use After Free

What is CVE-2026-2799?

Use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 148 and Thunderbird 148. The CVSS base score is 8.8 (High), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://bugzilla.mozilla.org/show_bug.cgi?id=2014551.

Am I affected?

Check the version of Firefox you are running and compare it against the Affected row above (See vendor advisory). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s firefox 2>/dev/null | grep -i ^Version
rpm -q firefox 2>/dev/null
command -v firefox >/dev/null && firefox --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*Firefox*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Firefox" 2>$null

How to fix CVE-2026-2799

Upgrade Firefox to a patched build: Firefox 148; Thunderbird 148. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade firefox
dpkg -s firefox | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh firefox -y
# or for older releases:
sudo yum update firefox -y
rpm -q firefox

SUSE / openSUSE


sudo zypper refresh
sudo zypper update firefox
rpm -q firefox

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-2799 affecting Firefox
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-2799-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detect installed version"
    $pkg = Get-Package -Name "*Firefox*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\firefox-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Firefox" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to Firefox 148; Thunderbird 148"
    winget upgrade --name "Firefox" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "Firefox"
    Write-Host "Patch applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-2799 affecting Firefox
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-2799-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s firefox 2>/dev/null | grep -i ^Version || echo "firefox not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q firefox || echo "firefox not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-2799-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/firefox /etc/firefox.d /etc/firefox.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: Firefox 148; Thunderbird 148)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y firefox
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y firefox
elif command -v yum >/dev/null; then
    yum update -y firefox
elif command -v zypper >/dev/null; then
    zypper --non-interactive update firefox
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s firefox 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q firefox
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)


# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)


New-NetFirewallRule -DisplayName "Block-CVE-2026-2799" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-2799"

Disable the affected service (Linux)


sudo systemctl stop firefox 2>/dev/null || true
sudo systemctl disable firefox 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.


# Linux
dpkg -s firefox 2>/dev/null | grep -i ^Version
rpm -q firefox 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true


# Windows
Get-Package -Name "*Firefox*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Firefox 148; Thunderbird 148. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-2799 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-2799?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-2799?

8.8 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.

Where is the official advisory for CVE-2026-2799?

The vendor advisory is at https://bugzilla.mozilla.org/show_bug.cgi?id=2014551. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-2799.

References

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*