⚠ 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 8.7

How to Fix CVE-2026-4211: D-Link DNS-1550-04 local_backup_mgr.cgi Local_Backup_Info stack-based overflow

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 8.7, High
Actively exploited?No
AffectedD-link DNS-120 (20260205); D-link DNR-202L (20260205); D-link DNS-315L (20260205); D-link DNS-320 (20260205)
Fixed inSee vendor advisory
Type (CWE)CWE-121: Stack-based Buffer Overflow

What is CVE-2026-4211?

A weakness has been identified in D-Link DNS-120, DNR-202L, DNS-315L, DNS-320, DNS-320L, DNS-320LW, DNS-321, DNR-322L, DNS-323, DNS-325, DNS-326, DNS-327L, DNR-326, DNS-340L, DNS-343, DNS-345, DNS-726-4, DNS-1100-4, DNS-1200-05 and DNS-1550-04 up to 20260205. Affected by this issue is the function Local_Backup_Info of the file /cgi-bin/local_backup_mgr.cgi. This manipulation of the argument f_idx causes stack-based buffer overflow.

In practical terms, a successful attacker gets memory corruption that can lead to code execution or a crash. There is no confirmed in-the-wild exploitation listed in CISA's KEV catalog at the time of writing, but the CVSS rating still warrants prompt patching.

Am I affected?

You're affected if you run D-link DNS-120 at any version in the Affected row above. Use these probes to find your installed build:


# Confirm the installed version via your package manager
dpkg -l | grep -i dns-120   # Debian/Ubuntu
rpm -qa | grep -i dns-120   # RHEL/CentOS/Rocky

How to fix CVE-2026-4211

The primary fix is to upgrade to the patched build listed in the Fixed in row above (See vendor advisory). Pick the platform that matches your install and run the commands below.

Consumer / SOHO router (web admin)

Sign in to the router admin UI at its LAN address, navigate to System / Administration > Firmware Upgrade, upload the patched firmware file from the vendor support portal, and apply. The router reboots automatically after a successful flash.

PowerShell script (Windows) - detect, back up, upgrade, verify, log


# Vendor advisory: https://www.dlink.com/
# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\DNS120-Patch-CVE-2026-4211.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2026-4211 remediation for D-link DNS-120"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*DNS-120*' } |
    Select-Object -First 1 -ExpandProperty Version)
Write-Log "Detected version: $installed"

if (-not $installed) {
    Write-Log "Product not installed on this host; nothing to do."
    return
}
if ([version]$installed -ge [version]'<patched-version>') {
    Write-Log "Already at fixed version $installed; no action needed."
    return
}

# 2. Backup configuration to a timestamped folder
$backup = "$env:ProgramData\DNS120-Backup-$(Get-Date -Format yyyyMMdd-HHmm)"
New-Item -ItemType Directory -Path $backup -Force | Out-Null
# Adjust the source path to match your install
$src = "$env:ProgramFiles\D-link\DNS-120"
if (Test-Path $src) { Copy-Item -Path $src -Destination $backup -Recurse -Force }
Write-Log "Backed up config to $backup"

# 3. Apply the patched installer (place the verified file on a share or staging path)
$installer = "$env:TEMP\DNS120-<patched-version>.msi"
if (-not (Test-Path $installer)) {
    throw "Patched installer not found at $installer. Stage it from your software repo first."
}
Start-Process msiexec.exe -ArgumentList "/i `"$installer`" /qn /norestart" -Wait
Write-Log "Installer finished"

# 4. Verify
$verify = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*DNS-120*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$verify -ge [version]'<patched-version>') {
    Write-Log "SUCCESS: now at $verify (>= <patched-version>)"
} else {
    Write-Log "FAILURE: still at $verify after install"
    exit 1
}

Bash script (Linux) - detect, back up, upgrade, verify, log


# Vendor advisory: https://www.dlink.com/
#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/dns-120-patch-cve-2026-4211.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2026-4211 remediation for D-link DNS-120"

# 1. Detect installed version (works for deb and rpm packages)
if command -v dpkg >/dev/null && dpkg -s dns-120 >/dev/null 2>&1; then
    CURRENT=$(dpkg-query -W -f='${Version}' dns-120)
    PKG_MGR=apt
elif command -v rpm >/dev/null && rpm -q dns-120 >/dev/null 2>&1; then
    CURRENT=$(rpm -q --queryformat '%{VERSION}' dns-120)
    PKG_MGR=dnf
else
    log "dns-120 not installed via apt or rpm; check your package manager or vendor instructions."
    exit 0
fi
log "Detected: dns-120=$CURRENT (manager=$PKG_MGR)"

# 2. Backup config
BACKUP=/var/backups/dns-120-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/dns-120 /etc/${pkg%%-*} ; do
    [ -d "$d" ] && cp -a "$d" "$BACKUP/" && log "Backed up $d to $BACKUP"
done

# 3. Upgrade
if [ "$PKG_MGR" = apt ]; then
    sudo apt-get update -y
    sudo apt-get install --only-upgrade -y dns-120
else
    sudo dnf upgrade --security -y dns-120
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' dns-120)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' dns-120)
fi
log "After upgrade: $NEW"

# Optionally compare against <patched-version> with dpkg --compare-versions or sort -V
log "Done. Restart the affected service if the package install did not."

If you can't patch immediately

These are runnable hardening commands. They reduce blast radius but they're not a replacement for the vendor patch.

No official vendor workaround is published for this CVE; patching is the only documented fix. The runnable hardening below is generic defense in depth, not a substitute for the patch.

Restrict the affected service to trusted networks (Linux):


# Vendor advisory: https://www.dlink.com/
# Replace <port> with the affected service port and 10.0.0.0/24 with your admin subnet
sudo iptables -A INPUT -p tcp --dport <port> -s 10.0.0.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport <port> -j DROP

Windows Firewall equivalent:


# Vendor advisory: https://www.dlink.com/
New-NetFirewallRule -DisplayName 'Allow affected service from admin subnet' \
    -Direction Inbound -Action Allow -Protocol TCP -LocalPort <port> -RemoteAddress 10.0.0.0/24
New-NetFirewallRule -DisplayName 'Block affected service from everywhere else' \
    -Direction Inbound -Action Block -Protocol TCP -LocalPort <port>

How to verify the fix worked

Run the version probe again and confirm the running build matches the Fixed in row above.


# Confirm the running build matches the patched version listed by the vendor
# Example for Linux package installs:
dpkg -l | grep -i "dns-120"   # Debian/Ubuntu
rpm -qa | grep -i "dns-120"   # RHEL/CentOS/Rocky

Expected output: the package version should meet or exceed the patched build.

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2026-4211 has cleared. Sweep your logs for the indicators of compromise listed in the vendor or CISA advisory, especially if the system was internet-reachable during the disclosure window.

Frequently asked questions

Is CVE-2026-4211 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

How severe is CVE-2026-4211?

CVSS rates it 8.7 (High). Use that score to set your patch priority next to the other items in your queue.

Do I have to take DNS-120 offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

What if my vulnerability scanner still flags CVE-2026-4211 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

References

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