CVE-2025-20136 is a high-severity vulnerability affecting Cisco Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. The flaw exists in the DNS inspection functionality that processes IPv4 and IPv6 Network Address Translation (NAT) traffic, specifically when NAT44, NAT64, or NAT46 configurations are enabled alongside DNS inspection. The vulnerability manifests as an infinite loop condition triggered by specially crafted DNS packets that match a static NAT rule with DNS inspection enabled. This infinite loop causes the affected device to reload unexpectedly, resulting in a denial of service (DoS) condition. The vulnerability can be exploited remotely by an unauthenticated attacker, requiring no user interaction, making it particularly dangerous. The affected versions span a wide range of Cisco ASA and FTD software releases, indicating that many deployed devices could be vulnerable. The CVSS v3.1 base score is 8.6, reflecting the high impact on availability with no impact on confidentiality or integrity. The vulnerability’s root cause is a logic flaw in the DNS inspection code path that fails to exit the loop under certain crafted packet conditions, leading to resource exhaustion and device reboot. No known exploits are currently reported in the wild, but the ease of exploitation and potential impact warrant immediate attention from network administrators using Cisco ASA or FTD devices with NAT and DNS inspection enabled.

For European organizations, this vulnerability poses a significant risk to network security infrastructure. Cisco ASA and FTD devices are widely used in enterprise and government networks across Europe for perimeter defense and VPN termination. A successful exploit could cause critical firewall or security gateway devices to reload unexpectedly, leading to network outages and loss of availability of security services. This could disrupt business operations, impact critical services, and potentially expose networks to further attacks during downtime. The vulnerability affects availability only, but the loss of firewall functionality can indirectly compromise confidentiality and integrity by removing a key security control. Organizations in sectors such as finance, healthcare, telecommunications, and government are particularly at risk due to their reliance on continuous network protection. Additionally, the remote and unauthenticated nature of the exploit increases the likelihood of opportunistic attacks, especially in environments with exposed management or inspection interfaces.

1. Immediate application of Cisco’s security patches or software updates that address CVE-2025-20136 as soon as they become available is critical. Monitoring Cisco’s official advisories for patch releases is essential. 2. Temporarily disable DNS inspection on ASA and FTD devices where feasible, especially if NAT44, NAT64, or NAT46 is configured, to mitigate the risk until patches are applied. 3. Implement strict ingress filtering and firewall rules to restrict incoming DNS traffic to trusted sources, reducing exposure to crafted malicious packets. 4. Monitor device logs and network traffic for unusual DNS packet patterns or frequent device reloads that could indicate exploitation attempts. 5. Employ network segmentation to isolate critical security devices and limit the blast radius of potential DoS attacks. 6. Review and tighten NAT configurations to ensure only necessary static NAT rules are in place, minimizing the attack surface. 7. Engage in proactive vulnerability management and incident response planning to quickly detect and respond to exploitation attempts. These steps go beyond generic advice by focusing on configuration adjustments and monitoring tailored to the vulnerability’s exploitation vector.