How to Fix CVE-2026-20091: Cross-site scripting in Cisco Firepower Extensible Operating System (FXOS)

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

CVE-2026-20091 is a cross-site scripting in Cisco Cisco Firepower Extensible Operating System (FXOS). The fix is to apply the vendor patch noted below.

What is CVE-2026-20091?

A vulnerability in the web-based management interface of Cisco FXOS Software and Cisco UCS Manager Software could allow an authenticated, remote attacker to conduct a stored cross-site scripting (XSS) attack against a user of the interface.

This vulnerability is due to insufficient validation of user-supplied input by the web-based management interface of an affected system. An attacker could exploit this vulnerability by injecting malicious data into specific pages of the interface. A successful exploit could allow the attacker to execute arbitrary script code in the context of the affected interface or access sensitive, browser-based information. To exploit this vulnerability, the attacker must have valid credentials for a user account with the role of Administrator or AAA Administrator.  The CVSS base score is 4.8 (Medium). The official advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ucsfxosxss-7skVE8Zv.

Am I affected?

Check the version of Cisco Firepower Extensible Operating System (FXOS) you are running and compare it against the Affected row above (Cisco Firepower Extensible Operating System (FXOS) 2.14.1.131; Cisco Firepower Extensible Operating System (FXOS) 2.14.1.143; Cisco Firepower Extensible Operating System (FXOS) 2.14.1.163; Cisco Firepower Extensible Operating System (FXOS) 2.14.1.167; Cisco Firepower Extensible Operating System (FXOS) 2.16.0.128; Cisco Secure Firewall Adaptive Security Appliance (ASA) Software 9.12.2). 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-firepower-extensible-operating-system-fxos 2>/dev/null | grep -i ^Version
rpm -q cisco-firepower-extensible-operating-system-fxos 2>/dev/null
command -v cisco-firepower-extensible-operating-system-fxos >/dev/null && cisco-firepower-extensible-operating-system-fxos --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*Cisco Firepower Extensible Operating System (FXOS)*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Cisco Firepower Extensible Operating System (FXOS)" 2>$null

How to fix CVE-2026-20091

Upgrade Cisco Firepower Extensible Operating System (FXOS) 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-firepower-extensible-operating-system-fxos
dpkg -s cisco-firepower-extensible-operating-system-fxos | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh cisco-firepower-extensible-operating-system-fxos -y
# or for older releases:
sudo yum update cisco-firepower-extensible-operating-system-fxos -y
rpm -q cisco-firepower-extensible-operating-system-fxos

SUSE / openSUSE

sudo zypper refresh
sudo zypper update cisco-firepower-extensible-operating-system-fxos
rpm -q cisco-firepower-extensible-operating-system-fxos

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-20091 affecting Cisco Firepower Extensible Operating System (FXOS)
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-20091-fix-$(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"
    $pkg = Get-Package -Name "*Cisco Firepower Extensible Operating System (FXOS)*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\cisco-firepower-extensible-operating-system-fxos-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Cisco Firepower Extensible Operating System (FXOS)" -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 Firepower Extensible Operating System (FXOS)" --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 Firepower Extensible Operating System (FXOS)"
    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-2026-20091 affecting Cisco Firepower Extensible Operating System (FXOS)
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-20091-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-firepower-extensible-operating-system-fxos 2>/dev/null | grep -i ^Version || echo "cisco-firepower-extensible-operating-system-fxos not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q cisco-firepower-extensible-operating-system-fxos || echo "cisco-firepower-extensible-operating-system-fxos not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-20091-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/cisco-firepower-extensible-operating-system-fxos /etc/cisco-firepower-extensible-operating-system-fxos.d /etc/cisco-firepower-extensible-operating-system-fxos.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-firepower-extensible-operating-system-fxos
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y cisco-firepower-extensible-operating-system-fxos
elif command -v yum >/dev/null; then
    yum update -y cisco-firepower-extensible-operating-system-fxos
elif command -v zypper >/dev/null; then
    zypper --non-interactive update cisco-firepower-extensible-operating-system-fxos
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s cisco-firepower-extensible-operating-system-fxos 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q cisco-firepower-extensible-operating-system-fxos
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-2026-20091" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-20091"

Disable the affected service (Linux)

sudo systemctl stop cisco-firepower-extensible-operating-system-fxos 2>/dev/null || true
sudo systemctl disable cisco-firepower-extensible-operating-system-fxos 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.

How to verify the fix worked

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-firepower-extensible-operating-system-fxos 2>/dev/null | grep -i ^Version
rpm -q cisco-firepower-extensible-operating-system-fxos 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 Firepower Extensible Operating System (FXOS)*" | 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.

Frequently asked questions

Related fixes

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

• How to Fix CVE-2026-20056: Code Injection RCE in Cisco Secure Web Appliance — Code Injection RCE in Cisco Secure Web Appliance
• How to Fix CVE-2026-20117: Multiple Cisco Contact Center Products Cross-Site Scripting Vulnerabilities — Multiple Cisco Contact Center Products Cross-Site Scripting Vulnerabilities
• How to Fix CVE-2026-20126: Incorrect use of privileged apis in Cisco Catalyst SD-WAN Manager , Incorrect use of privileged apis in Cisco Catalyst SD-WAN Manager
• How to Fix CVE-2026-20062: Incorrect execution-assigned permissions in Cisco Secure Firewall Adaptive. , Incorrect execution-assigned permissions in Cisco Secure Firewall Adaptive.
• How to Fix CVE-2026-20021: Missing release of memory after effective lifetime in Cisco Secure Firewall. , Missing release of memory after effective lifetime in Cisco Secure Firewall.

Is CVE-2026-20091 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-2026-20091?

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-2026-20091?

4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N.

Where is the official advisory for CVE-2026-20091?

The vendor advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ucsfxosxss-7skVE8Zv. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-20091.

References

• Official vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-ucsfxosxss-7skVE8Zv
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20091
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

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