How to Fix CVE-2026-20100: Buffer overflow in Cisco Secure Firewall Adaptive Security Appliance (ASA) Software

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

CVE-2026-20100 is a buffer overflow in Cisco Cisco Secure Firewall Adaptive Security Appliance (ASA) Software. The fix is to apply the vendor patch noted below.

What is CVE-2026-20100?

A vulnerability in the LUA interperter of the Remote Access SSL VPN feature of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Secure Firewall Threat Defense (FTD) Software could allow an authenticated, remote attacker with a valid VPN connection to cause the device to reload unexpectedly, resulting in a denial of service (DoS) condition. This does not affect the management or MUS interfaces.

This vulnerability is due to trusting user input without validation in the LUA interprerter. An attacker could exploit this vulnerability by sending crafted HTTP packets to the Remote Access SSL VPN server. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition. The CVSS base score is 7.7 (High), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-m9sx6MbC.

What you'll see

Check the version of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software you are running and compare it against the Affected row above (Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.1.3; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.4; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2.5). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

# Linux package check
dpkg -s cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null | grep -i ^Version
rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null
command -v cisco-secure-firewall-adaptive-security-appliance-asa-software >/dev/null && cisco-secure-firewall-adaptive-security-appliance-asa-software --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*Cisco Secure Firewall Adaptive Security Appliance (ASA) Software*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Cisco Secure Firewall Adaptive Security Appliance (ASA) Software" 2>$null

How to fix CVE-2026-20100

Upgrade Cisco Secure Firewall Adaptive Security Appliance (ASA) Software to a patched build: See vendor advisory. 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 cisco-secure-firewall-adaptive-security-appliance-asa-software
dpkg -s cisco-secure-firewall-adaptive-security-appliance-asa-software | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh cisco-secure-firewall-adaptive-security-appliance-asa-software -y
# or for older releases:
sudo yum update cisco-secure-firewall-adaptive-security-appliance-asa-software -y
rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software

SUSE / openSUSE

sudo zypper refresh
sudo zypper update cisco-secure-firewall-adaptive-security-appliance-asa-software
rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-20100 affecting Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-20100-fix-$(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"
    $pkg = Get-Package -Name "*Cisco Secure Firewall Adaptive Security Appliance (ASA) Software*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\cisco-secure-firewall-adaptive-security-appliance-asa-software-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Cisco Secure Firewall Adaptive Security Appliance (ASA) Software" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "Cisco Secure Firewall Adaptive Security Appliance (ASA) Software" --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 "Cisco Secure Firewall Adaptive Security Appliance (ASA) Software"
    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-2026-20100 affecting Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-20100-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 cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null | grep -i ^Version || echo "cisco-secure-firewall-adaptive-security-appliance-asa-software not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software || echo "cisco-secure-firewall-adaptive-security-appliance-asa-software not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-20100-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/cisco-secure-firewall-adaptive-security-appliance-asa-software /etc/cisco-secure-firewall-adaptive-security-appliance-asa-software.d /etc/cisco-secure-firewall-adaptive-security-appliance-asa-software.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: See vendor advisory)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y cisco-secure-firewall-adaptive-security-appliance-asa-software
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y cisco-secure-firewall-adaptive-security-appliance-asa-software
elif command -v yum >/dev/null; then
    yum update -y cisco-secure-firewall-adaptive-security-appliance-asa-software
elif command -v zypper >/dev/null; then
    zypper --non-interactive update cisco-secure-firewall-adaptive-security-appliance-asa-software
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software
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-2026-20100" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-20100"

Disable the affected service (Linux)

sudo systemctl stop cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null || true
sudo systemctl disable cisco-secure-firewall-adaptive-security-appliance-asa-software 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.

The repair

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 cisco-secure-firewall-adaptive-security-appliance-asa-software 2>/dev/null | grep -i ^Version
rpm -q cisco-secure-firewall-adaptive-security-appliance-asa-software 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 "*Cisco Secure Firewall Adaptive Security Appliance (ASA) Software*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

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

Is CVE-2026-20100 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-2026-20100?

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-2026-20100?

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

Where is the official advisory for CVE-2026-20100?

The vendor advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-m9sx6MbC. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-20100.

References

• Official vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-m9sx6MbC
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20100
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Written by Sai Kiran Pandrala on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.

Related fixes

Other defects in the same area that deserve attention during this patch cycle:

• How to Fix CVE-2026-20171: Denial of Service in Cisco NX-OS Software
• How to Fix CVE-2026-20206: Command Injection in Cisco ThousandEyes Enterprise Agent
• How to Fix CVE-2026-20007: Improper access control in Cisco Secure Firewall Threat Defense (FTD) Software
• How to Fix CVE-2026-20016: Improper neutralization of argument delimiters in a command ('argument injection').
• How to Fix CVE-2026-20160: Cisco Smart Software Manager On-Prem Arbitrary Command Execution