CVE-2026-20039
📋 TL;DR
An unauthenticated remote attacker can cause Cisco ASA/FTD firewall devices to reload by sending crafted HTTP requests to the VPN web server, resulting in denial of service. This affects Cisco Secure Firewall ASA Software and FTD Software with vulnerable VPN configurations. The vulnerability stems from ineffective memory management in the VPN web server component.
💻 Affected Systems
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
- Cisco Secure Firewall Threat Defense (FTD) Software
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Complete service disruption of firewall/VPN services, potentially affecting all network traffic through the device and requiring manual intervention to restore service.
Likely Case
Intermittent firewall/VPN service outages causing connectivity issues for users and applications relying on the affected device.
If Mitigated
Minimal impact if VPN web server is disabled or properly segmented from untrusted networks.
🎯 Exploit Status
Exploitation requires sending crafted HTTP requests to the VPN web server; no authentication required.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Cisco advisory for specific fixed versions
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-dos-SpOFF2Re
Restart Required: Yes
Instructions:
1. Review Cisco advisory for affected versions. 2. Download and apply appropriate firmware update. 3. Reload device to apply patch. 4. Verify VPN functionality post-update.
🔧 Temporary Workarounds
Disable VPN Web Server
allTemporarily disable the VPN web server if not required for operations
no webvpn
write memory
Restrict VPN Access
allLimit VPN web server access to trusted networks only using ACLs
access-list VPN-ACL permit ip trusted-network any
webvpn enable outside
access-group VPN-ACL in interface outside
🧯 If You Can't Patch
- Implement network segmentation to restrict access to VPN web server from untrusted networks
- Deploy intrusion prevention system (IPS) with signatures for this vulnerability
🔍 How to Verify
Check if Vulnerable:
Check if VPN web server is enabled: 'show running-config | include webvpn' and verify version against Cisco advisoryCheck Version:
show version | include VersionVerify Fix Applied:
Verify firmware version after update: 'show version' and confirm it matches fixed version from Cisco advisory📡 Detection & Monitoring
Log Indicators:
- Multiple connection attempts to VPN web server from single source
- Device reload events in system logs
- High memory usage alerts on VPN process
Network Indicators:
- Unusual HTTP traffic patterns to VPN web server port
- Multiple crafted HTTP requests from single IP
SIEM Query:
source="cisco-asa" (event_id=722041 OR message="%ASA-6-722041") OR (message="Reloading")