CVE-2026-20073
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to bypass access controls on Cisco ASA and FTD firewalls by sending traffic that should be blocked. The bypass occurs when devices joining a cluster run out of memory during access control rule replication. Organizations using affected Cisco firewall versions in cluster configurations are at risk.
💻 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
Attackers completely bypass firewall protections, accessing protected internal networks and systems that should be inaccessible from external sources.
Likely Case
Attackers bypass specific access control rules, potentially accessing some protected resources but not necessarily full network compromise.
If Mitigated
With proper network segmentation and defense-in-depth, impact is limited to specific segments behind the affected firewall.
🎯 Exploit Status
Requires specific timing during cluster join operations when memory is exhausted
🛠️ 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-aclbypass-dos-CVxVRSvQ
Restart Required: Yes
Instructions:
1. Review Cisco advisory for affected versions. 2. Download and apply appropriate patch from Cisco. 3. Reboot affected devices. 4. Verify patch application and functionality.
🔧 Temporary Workarounds
Monitor Cluster Join Operations
allMonitor memory usage during cluster join operations and avoid joining clusters during high-traffic periods
Increase Available Memory
allEnsure sufficient memory is available before initiating cluster join operations
🧯 If You Can't Patch
- Implement additional network segmentation behind affected firewalls
- Deploy intrusion detection/prevention systems to monitor for bypass attempts
🔍 How to Verify
Check if Vulnerable:
Check device version against Cisco advisory and verify cluster configurationCheck Version:
show version (Cisco ASA/FTD CLI)Verify Fix Applied:
Verify patch version is installed and test access control rules functionality📡 Detection & Monitoring
Log Indicators:
- Memory exhaustion events during cluster join
- Access control rule replication failures
- Unexpected traffic allowed through firewall
Network Indicators:
- Traffic matching denied rules successfully traversing firewall
- Increased traffic to protected segments during cluster operations
SIEM Query:
Search for 'memory exhaustion' AND 'cluster join' OR 'ACL replication failure' in firewall logs