How to Fix CVE-2016-9079: Use After Free in Firefox

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*By Sai Kiran Pandrala*

Patch immediately. CISA's Known Exploited Vulnerabilities catalog lists this CVE, which means active exploitation has been confirmed. CISA KEV entry added 2023-06-22, federal due date 2023-07-13.

What is CVE-2016-9079?

CVE-2016-9079 is a use after free in Firefox from Mozilla. A use-after-free vulnerability in SVG Animation has been discovered. An exploit built on this vulnerability has been discovered in the wild targeting Firefox and Tor Browser users on Windows. This vulnerability affects Firefox < 50.0.2, Firefox ESR < 45.5.1, and Thunderbird < 45.5.1.

Why this CVE matters

This CVE sits on CISA's Known Exploited Vulnerabilities catalog, which only happens after active exploitation is observed in the wild. The use after free class of flaw gives attackers a reliable foothold against vulnerable instances of Firefox. If your deployment matches the affected versions, treat any window of unpatched exposure as compromise-likely and review logs accordingly.

Am I affected?

Run the version check that matches your platform. If the installed build sits inside the affected range from the table above, the fix applies to you.

# Linux package check
dpkg -s firefox 2>/dev/null | grep -i version    # Debian / Ubuntu
rpm -q firefox 2>/dev/null                       # RHEL / Rocky

# Windows version check
Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'Firefox' } | Select-Object Name,Version

How to fix CVE-2016-9079

Apply the patched build the vendor names in the advisory. The commands below are starting points keyed to common platforms - adapt the package name and target version to your environment.

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade firefox
# verify the package version matches the fixed release in the advisory
dpkg -s firefox | grep -i version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh firefox -y
rpm -q firefox

Container image

# Vendor advisory: https://www.debian.org/security/2016/dsa-3730
# Pull the patched base image and rebuild
docker pull <your-registry>/firefox:<patched-tag>
docker build -t <your-app>:patched .
docker stop <your-app> && docker rm <your-app>
docker run -d --name <your-app> <your-app>:patched

Windows (PowerShell, run as administrator)

# Vendor advisory: https://www.debian.org/security/2016/dsa-3730
# Make sure Windows Update is current. For monthly rollups this is the safest path.
Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -Confirm:$false
Import-Module PSWindowsUpdate
Get-WindowsUpdate -KBArticleID <KB-from-advisory>
Install-WindowsUpdate -KBArticleID <KB-from-advisory> -AcceptAll -AutoReboot

# Confirm the KB landed
Get-HotFix | Where-Object { $_.HotFixID -eq 'KB<id>' }

# Or, for an MSU file downloaded from the Microsoft Update Catalog:
wusa.exe C:\Patches\windows10.0-kb<id>-x64.msu /quiet /norestart
shutdown /r /t 60

PowerShell detect/upgrade/verify/log (Windows)

# CVE-2016-9079 remediation runner — adapt the version checks to your environment.
$log = "C:\Logs\CVE-2016-9079-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
function Write-Log($msg) { "$(Get-Date -Format s) $msg" | Out-File $log -Append }

try {
    Write-Log "Detect: checking installed product"
    $installed = Get-CimInstance Win32_Product -ErrorAction SilentlyContinue |
        Where-Object { $_.Name -match 'Firefox' }
    if (-not $installed) { Write-Log "Product not installed; nothing to do"; return }
    Write-Log "Found version $($installed.Version)"

    Write-Log "Backup: copying program files and registry hive"
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\CVE-2016-9079-$stamp"
    New-Item -ItemType Directory -Force -Path $backup | Out-Null
    Copy-Item $installed.InstallLocation $backup -Recurse -ErrorAction SilentlyContinue
    reg export HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall "$backup\uninstall.reg" /y | Out-Null

    Write-Log "Upgrade: install patched build via vendor MSI / Windows Update"
    # Example MSI:  Start-Process msiexec.exe -ArgumentList '/i C:\Patches\Firefox-patched.msi /qn /norestart' -Wait
    Install-WindowsUpdate -AcceptAll -AutoReboot -ErrorAction SilentlyContinue

    Write-Log "Verify: re-reading product version"
    $after = Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'Firefox' }
    Write-Log "Post-patch version: $($after.Version)"
    if ($after.Version -ne $installed.Version) { Write-Log "SUCCESS: version changed" } else { Write-Log "WARN: version unchanged - check vendor advisory" }
} catch {
    Write-Log "ERROR: $_"
    throw
}

Bash detect/upgrade/verify/log (Linux)

#!/usr/bin/env bash
# CVE-2016-9079 remediation runner. Re-runnable, exits non-zero on failure.
set -euo pipefail
log() { printf '%s %s\n' "$(date -Is)" "$*" | tee -a /var/log/cve-2016-9079-fix.log; }

log "Detect: current firefox version"
if command -v dpkg >/dev/null 2>&1; then
    current=$(dpkg-query -W -f='${Version}' firefox 2>/dev/null || echo "not-installed")
elif command -v rpm >/dev/null 2>&1; then
    current=$(rpm -q --qf '%{VERSION}-%{RELEASE}' firefox 2>/dev/null || echo "not-installed")
else
    current="unknown"
fi
log "Current: $current"

log "Backup: snapshotting config"
backup="/var/backups/cve-2016-9079-$(date +%Y%m%d-%H%M)"
mkdir -p "$backup"
[ -d /etc/firefox ] && cp -a /etc/firefox "$backup/" || true

log "Upgrade: applying vendor patch"
if command -v apt-get >/dev/null 2>&1; then
    sudo apt-get update -qq
    sudo apt-get install -y --only-upgrade firefox
elif command -v dnf >/dev/null 2>&1; then
    sudo dnf upgrade -y firefox
elif command -v yum >/dev/null 2>&1; then
    sudo yum update -y firefox
fi

log "Verify: re-reading firefox version"
if command -v dpkg >/dev/null 2>&1; then
    after=$(dpkg-query -W -f='${Version}' firefox)
else
    after=$(rpm -q --qf '%{VERSION}-%{RELEASE}' firefox)
fi
log "After: $after"

if [ "$after" != "$current" ]; then
    log "SUCCESS: firefox upgraded"
else
    log "WARN: version unchanged. Confirm the patched build is in your repository."
    exit 1
fi

After the upgrade, restart any service that loads the patched binary so the new code is actually running.

If you can't patch immediately

Patching is the only durable fix. These mitigations cut exposure while the change window is scheduled, they do not remove the vulnerability.

Windows: disable the vulnerable feature via registry

# Vendor advisory: https://www.debian.org/security/2016/dsa-3730
# Replace <Component> / <Setting> with the exact key from the vendor advisory.
# Always export the key first so you can roll back.
reg export "HKLM\SOFTWARE\Vendor\firefox" "C:\Backup\preFix-CVE-2016-9079.reg" /y
Set-ItemProperty -Path "HKLM:\SOFTWARE\Vendor\firefox" -Name "Disable<Setting>" -Value 1 -Type DWord
Restart-Service -Name "<service-name>" -Force

How to verify the fix worked

After applying the patched build, confirm the version string matches the fixed release named in the Mozilla advisory.

dpkg -s firefox | grep -i version       # Debian / Ubuntu
rpm -q firefox                          # RHEL / Rocky

Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Run an authenticated vulnerability scan with a current signature set and confirm the scanner no longer flags CVE-2016-9079. For internet-facing deployments that were unpatched during the disclosure window, review logs for the affected endpoints over the full exposure period and rotate any credentials the vulnerable process could touch.

Frequently asked questions

Is CVE-2016-9079 being exploited in the wild?

Yes. CISA added CVE-2016-9079 to the Known Exploited Vulnerabilities catalog, which means active exploitation has been confirmed.

Will a firewall rule or WAF signature fully mitigate CVE-2016-9079?

No. Network-layer filters slow opportunistic scanners and block a subset of payloads, but a focused attacker who knows the bug will work around them. The vendor patch is the only durable fix.

Do I need to assume compromise if the affected service was internet-facing and unpatched?

For a CVE that CISA confirms is under active exploitation, yes. Review logs for the affected endpoints over the entire exposure window, rotate credentials the vulnerable process could read, and look for unexpected accounts, scheduled tasks, or outbound connections.

References

• Official vendor advisory: https://www.debian.org/security/2016/dsa-3730
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2016-9079
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• CISA KEV record: https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2016-9079
• Additional reference: http://rhn.redhat.com/errata/RHSA-2016-2843.html
• Additional reference: https://security.gentoo.org/glsa/201701-35
• Additional reference: http://www.securitytracker.com/id/1037370
• Additional reference: https://www.exploit-db.com/exploits/42327/

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*