CVE-2026-20100
📋 TL;DR
This vulnerability allows authenticated remote attackers with VPN access to cause Cisco ASA/FTD devices to crash and reload by sending specially crafted HTTP packets to the SSL VPN server. The issue affects Cisco Secure Firewall ASA Software and Secure Firewall Threat Defense (FTD) Software. Only the Remote Access SSL VPN feature is vulnerable, not management or MUS interfaces.
💻 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 denial of service for all VPN users and potential service disruption for network traffic passing through the firewall during reload, which could take several minutes.
Likely Case
Temporary VPN service disruption affecting remote users until the device completes reload, typically 2-5 minutes of downtime.
If Mitigated
No impact if VPN feature is disabled or proper input validation is implemented.
🎯 Exploit Status
Requires valid VPN authentication. Exploitation involves sending crafted HTTP packets to the VPN server.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Multiple fixed versions available - see Cisco advisory for specific versions
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-vpn-m9sx6MbC
Restart Required: Yes
Instructions:
1. Review Cisco advisory for specific fixed versions for your platform. 2. Download appropriate software from Cisco Software Center. 3. Backup current configuration. 4. Upgrade to fixed version following Cisco upgrade procedures. 5. Verify upgrade success and functionality.
🔧 Temporary Workarounds
Disable Remote Access SSL VPN
allTemporarily disable the vulnerable feature until patching can be completed
no webvpn
no enable outside
Restrict VPN Access
allLimit VPN access to trusted IP ranges only
access-list VPN-ACL permit ip <trusted-networks> any
access-group VPN-ACL in interface outside
🧯 If You Can't Patch
- Disable Remote Access SSL VPN feature entirely if not required
- Implement strict network segmentation and limit VPN access to minimum required users
🔍 How to Verify
Check if Vulnerable:
Check current ASA/FTD version and compare against affected versions in Cisco advisory. Verify if webvpn is enabled with 'show running-config | include webvpn'Check Version:
show version | include VersionVerify Fix Applied:
Verify software version after upgrade matches fixed version from Cisco advisory. Confirm webvpn functionality works without crashes.📡 Detection & Monitoring
Log Indicators:
- Unexpected device reloads
- VPN session disconnections
- LUA interpreter errors in system logs
- HTTP parsing errors
Network Indicators:
- Unusual HTTP traffic patterns to VPN endpoint
- Multiple connection attempts with malformed packets
SIEM Query:
source="asa" AND ("reload" OR "crash" OR "webvpn error")