CVE-2025-20135
📋 TL;DR
A memory exhaustion vulnerability in Cisco ASA and FTD DHCP clients allows adjacent attackers to cause denial of service by sending crafted DHCPv4 packets. This affects Cisco Secure Firewall ASA Software and Cisco Secure Firewall Threat Defense (FTD) Software. Exploitation requires adjacent network access and prevents new processes from starting until manual reboot.
💻 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 requiring manual reboot, affecting all services on the firewall until recovery.
Likely Case
Intermittent service degradation and availability issues during attack periods.
If Mitigated
Minimal impact with proper network segmentation and monitoring in place.
🎯 Exploit Status
Exploitation requires crafting DHCPv4 packets and adjacent network access. No authentication needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Refer to Cisco Security Advisory for fixed versions
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-dhcp-qj7nGs4N
Restart Required: No
Instructions:
1. Review Cisco Security Advisory for affected versions. 2. Upgrade to fixed software version. 3. No reboot required for patch application.
🔧 Temporary Workarounds
Disable DHCP client
allConfigure static IP addresses instead of DHCP to eliminate attack surface
configure terminal
interface <interface_name>
ip address <static_ip> <subnet_mask>
no ip address dhcp
Network segmentation
allIsolate DHCP server and client traffic to trusted segments only
🧯 If You Can't Patch
- Implement strict network access controls to limit DHCP traffic to trusted sources only
- Monitor for abnormal DHCP traffic patterns and memory usage spikes
🔍 How to Verify
Check if Vulnerable:
Check software version with 'show version' and compare against Cisco advisoryCheck Version:
show versionVerify Fix Applied:
Verify version is updated to fixed release and monitor for memory exhaustion events📡 Detection & Monitoring
Log Indicators:
- Memory exhaustion warnings
- DHCP client errors
- Process failure logs
Network Indicators:
- Unusual DHCP packet rates from single sources
- Malformed DHCP packets
SIEM Query:
source="cisco_asa" AND ("%ASA-4-733100" OR "DHCP" AND "memory")