⚠ 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 7.5 ⚠ ACTIVELY EXPLOITED — CISA KEV

How to Fix CVE-2018-0171: Improper Input Validation in Cisco IOS and IOS XE

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 7.5, High
Actively exploited?Yes, listed in CISA KEV (added 2021-11-03)
AffectedCisco IOS and IOS XE (Cisco IOS and IOS XE)
Fixed inSee vendor advisory
Type (CWE)CWE-20
⚠️ Patch immediately. CVE-2018-0171 is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03). Federal agencies had until 2022-05-03 to remediate.

What is CVE-2018-0171?

A vulnerability in the Smart Install feature of Cisco IOS Software and Cisco IOS XE Software could allow an unauthenticated, remote attacker to trigger a reload of an affected device, resulting in a denial of service (DoS) condition, or to execute arbitrary code on an affected device. The vulnerability is due to improper validation of packet data. An attacker could exploit this vulnerability by sending a crafted Smart Install message to an affected device on TCP port 4786.

In practical terms, a successful attacker gets remote code execution on the affected system. CISA has confirmed exploitation in the wild, so this is not a theoretical risk.

Am I affected?

You're affected if you run Cisco IOS and IOS XE at any version in the Affected row above. Use these probes to find your installed build:


show version | include Version

How to fix CVE-2018-0171

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.

Cisco IOS / IOS XE / IOS XR (CLI)


# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2
enable
# Confirm running image and version
show version | include Version
# Stage the patched image (use the verified build from the advisory, e.g. <patched-version>)
copy ftp://upload-server/cisco-ios-and-ios-xe-<patched-version>.bin flash:
# Make the new image the boot image
configure terminal
 boot system flash:cisco-ios-and-ios-xe-<patched-version>.bin
 end
write memory
reload

Cisco ASA / FTD (ASDM or CLI upgrade)


# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2
enable
show version
copy tftp://10.0.0.10/asa<patched-version>-smp-k8.bin disk0:
configure terminal
 boot system disk0:/asa<patched-version>-smp-k8.bin
 end
write memory
reload

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


# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2
# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\CiscoIOSandIOSXE-Patch-CVE-2018-0171.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2018-0171 remediation for Cisco Cisco IOS and IOS XE"

# 1. Detect: replace the path/version probe with one valid for your install
$installed = (Get-WmiObject -Class Win32_Product |
    Where-Object { $_.Name -like '*Cisco*' } |
    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\CiscoIOSandIOSXE-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\Cisco\Cisco IOS and IOS XE"
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\CiscoIOSandIOSXE-<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 { $_.Name -like '*Cisco*' } |
    Select-Object -First 1 -ExpandProperty Version)
if ([version]$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://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2
#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/cisco-ios-and-ios-xe-patch-cve-2018-0171.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2018-0171 remediation for Cisco Cisco IOS and IOS XE"

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

# 2. Backup config
BACKUP=/var/backups/cisco-ios-and-ios-xe-$(date +%Y%m%d-%H%M)
mkdir -p "$BACKUP"
for d in /etc/cisco-ios-and-ios-xe /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 cisco-ios-and-ios-xe
else
    sudo dnf upgrade --security -y cisco-ios-and-ios-xe
fi

# 4. Verify
if [ "$PKG_MGR" = apt ]; then
    NEW=$(dpkg-query -W -f='${Version}' cisco-ios-and-ios-xe)
else
    NEW=$(rpm -q --queryformat '%{VERSION}' cisco-ios-and-ios-xe)
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.

Block external access to the affected service

Apply a perimeter ACL so the exploitable port is only reachable from administrative subnets:


# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20180328-smi2
# Example Cisco ACL on the upstream router
access-list 110 permit tcp 10.0.0.0 0.0.0.255 host <appliance-ip> eq 443
access-list 110 deny   tcp any host <appliance-ip> eq 443 log
interface GigabitEthernet0/0
 ip access-group 110 in

Rate-limit and log the affected endpoint at the reverse proxy


limit_req_zone $binary_remote_addr zone=admin:10m rate=10r/m;
location /<vulnerable-path> {
    limit_req zone=admin burst=5 nodelay;
    access_log /var/log/nginx/admin-access.log combined;
    proxy_pass http://backend;
}

Rate-limit and watchdog the affected service

Linux:


# Drop traffic above 50 connections/second from a single source
sudo iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW -m limit --limit 50/s -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -m conntrack --ctstate NEW -j DROP

Set systemd to auto-restart the service on crash:


[Service]
Restart=always
RestartSec=5s

How to verify the fix worked

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


show version | include Version

Expected output: a line that includes the patched build (for example Version the patched build or higher).

Then re-run any vulnerability scanner you used previously and confirm the finding for CVE-2018-0171 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-2018-0171 being actively exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog, which means in-the-wild exploitation has been observed and confirmed.

How severe is CVE-2018-0171?

CVSS rates it 7.5 (High). Use that score to set your patch priority next to the other items in your queue.

Do I have to take Cisco IOS and IOS XE 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-2018-0171 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.*