How to Fix CVE-2020-3161: Improper Input Validation in Cisco IP phone

By the Sai Kiran Pandrala · Reviewed and edited by Sai Kiran Pandrala, Editor

⚠️ Patch immediately. CVE-2020-3161 is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03). Federal agencies had until 2022-05-03 to remediate.

What is CVE-2020-3161?

A vulnerability in the web server for Cisco IP Phones could allow an unauthenticated, remote attacker to execute code with root privileges or cause a reload of an affected IP phone, resulting in a denial of service (DoS) condition. The vulnerability is due to a lack of proper input validation of HTTP requests. An attacker could exploit this vulnerability by sending a crafted HTTP request to the web server of a targeted device.

In practical terms, a successful attacker gets a denial-of-service condition that crashes or hangs the affected service. CISA has confirmed exploitation in the wild, so this is not a theoretical risk.

Identify

You're affected if you run Cisco IP phone 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 cisco   # Debian/Ubuntu
rpm -qa | grep -i cisco   # RHEL/CentOS/Rocky

How to fix CVE-2020-3161

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.

Linux (Ubuntu / Debian)

# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-voip-phones-rce-dos-rB6EeRXs
sudo apt-get update
sudo apt-get install --only-upgrade cisco-ip-phone
# Confirm the installed version meets or exceeds <patched-version>
dpkg -s cisco-ip-phone | grep ^Version

Linux (RHEL / CentOS / Rocky)

sudo dnf upgrade --security cisco-ip-phone -y
rpm -q cisco-ip-phone

Windows (PowerShell, admin)

# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-voip-phones-rce-dos-rB6EeRXs
# Try winget first
winget upgrade --id 'Cisco.Cisco IP phone' --silent --accept-source-agreements --accept-package-agreements
# If winget does not know the product, download the patched installer from the vendor and:
Start-Process -FilePath "$env:TEMP\CiscoIPphone-<patched-version>.msi" -ArgumentList '/qn /norestart' -Wait

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

# Vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-voip-phones-rce-dos-rB6EeRXs
# Run as Administrator
$ErrorActionPreference = 'Stop'
$log = "$env:ProgramData\CiscoIPphone-Patch-CVE-2020-3161.log"
function Write-Log($msg) { "$(Get-Date -Format s)  $msg" | Tee-Object -FilePath $log -Append }

Write-Log "Starting CVE-2020-3161 remediation for Cisco Cisco IP phone"

# 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\CiscoIPphone-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 IP phone"
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\CiscoIPphone-<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-voip-phones-rce-dos-rB6EeRXs
#!/usr/bin/env bash
set -euo pipefail
LOG=/var/log/cisco-ip-phone-patch-cve-2020-3161.log
log()  { echo "$(date -Iseconds)  $*" | tee -a "$LOG"; }

log "Starting CVE-2020-3161 remediation for Cisco Cisco IP phone"

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

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

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

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

Resolve

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 "cisco"   # Debian/Ubuntu
rpm -qa | grep -i "cisco"   # 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-2020-3161 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-2020-3161 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-2020-3161?

CVSS rates it 9.8 (Critical). Use that score to set your patch priority next to the other items in your queue.

Do I have to take Cisco IP phone 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-2020-3161 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

• Official vendor advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-voip-phones-rce-dos-rB6EeRXs
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-3161
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Additional reference: http://packetstormsecurity.com/files/157265/Cisco-IP-Phone-11.7-Denial-Of-Service.html

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.

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• How to Fix CVE-2020-3259: Security Vulnerability in Cisco Adaptive Security Appliance (Asa) Software
• How to Fix CVE-2020-3433: Security Vulnerability in Cisco Anyconnect Secure Mobility Client
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