How to Fix CVE-2026-20031: Uncaught exception in Cisco Secure Endpoint

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

CVE-2026-20031 is a uncaught exception in Cisco Cisco Secure Endpoint. The fix is to apply the vendor patch noted below.

What is CVE-2026-20031?

A vulnerability in the HTML Cascading Style Sheets (CSS) module of ClamAV could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.

This vulnerability is due to improper error handling when splitting UTF-8 strings. An attacker could exploit this vulnerability by submitting a crafted HTML file to be scanned by ClamAV on an affected device. A successful exploit could allow the attacker to terminate the scanning process. The CVSS base score is 5.3 (Medium). The official advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-clamav-css-Fn4QSZ.

Am I affected?

Check the version of Cisco Secure Endpoint you are running and compare it against the Affected row above (Cisco Secure Endpoint 7.0.5; Cisco Secure Endpoint 6.2.19; Cisco Secure Endpoint 7.3.3; Cisco Secure Endpoint 7.2.13; Cisco Secure Endpoint 6.1.5; Cisco Secure Endpoint 6.3.1). 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-endpoint 2>/dev/null | grep -i ^Version
rpm -q cisco-secure-endpoint 2>/dev/null
command -v cisco-secure-endpoint >/dev/null && cisco-secure-endpoint --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*Cisco Secure Endpoint*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Cisco Secure Endpoint" 2>$null

How to fix CVE-2026-20031

Upgrade Cisco Secure Endpoint 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-endpoint
dpkg -s cisco-secure-endpoint | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh cisco-secure-endpoint -y
# or for older releases:
sudo yum update cisco-secure-endpoint -y
rpm -q cisco-secure-endpoint

SUSE / openSUSE

sudo zypper refresh
sudo zypper update cisco-secure-endpoint
rpm -q cisco-secure-endpoint

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-20031 affecting Cisco Secure Endpoint
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-20031-fix-$(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"
    $pkg = Get-Package -Name "*Cisco Secure Endpoint*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\cisco-secure-endpoint-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Cisco Secure Endpoint" -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 Endpoint" --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 Endpoint"
    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-2026-20031 affecting Cisco Secure Endpoint
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-20031-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-endpoint 2>/dev/null | grep -i ^Version || echo "cisco-secure-endpoint not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q cisco-secure-endpoint || echo "cisco-secure-endpoint not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-20031-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/cisco-secure-endpoint /etc/cisco-secure-endpoint.d /etc/cisco-secure-endpoint.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-endpoint
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y cisco-secure-endpoint
elif command -v yum >/dev/null; then
    yum update -y cisco-secure-endpoint
elif command -v zypper >/dev/null; then
    zypper --non-interactive update cisco-secure-endpoint
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s cisco-secure-endpoint 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q cisco-secure-endpoint
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-2026-20031" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-20031"

Disable the affected service (Linux)

sudo systemctl stop cisco-secure-endpoint 2>/dev/null || true
sudo systemctl disable cisco-secure-endpoint 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-secure-endpoint 2>/dev/null | grep -i ^Version
rpm -q cisco-secure-endpoint 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 Endpoint*" | 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-2026-20036: Os command injection in Cisco Unified Computing System (Managed) — Os command injection in Cisco Unified Computing System (Managed)
• How to Fix CVE-2026-20209: Cross-Site Scripting in Cisco Catalyst SD-WAN Manager — Cross-Site Scripting in Cisco Catalyst SD-WAN Manager
• How to Fix CVE-2026-20057: Divide by zero in Cisco Cyber Vision , Divide by zero in Cisco Cyber Vision
• How to Fix CVE-2026-20167: Improper Access Control in Cisco IoT Field Network Director (IoT-FND) , Improper Access Control in Cisco IoT Field Network Director (IoT-FND)
• How to Fix CVE-2026-20009: Improper neutralization of special elements in Cisco Secure Firewall Adaptive. , Improper neutralization of special elements in Cisco Secure Firewall Adaptive.

Is CVE-2026-20031 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-2026-20031?

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-2026-20031?

5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.

Where is the official advisory for CVE-2026-20031?

The vendor advisory is at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-clamav-css-Fn4QSZ. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-20031.

References

• Official vendor advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-clamav-css-Fn4QSZ
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-20031
• 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.*