CVE-2026-20250 in Secure Firewall Adaptive Security Appliance Software
Summary
by MITRE • 09/16/2026
A vulnerability in Datagram TLS (DTLS) message handling of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for Cisco Secure Firewall 3100 Series and 4200 Series devices could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.
This vulnerability is due to improper resource management when processing certain DTLS messages. An attacker could exploit this vulnerability by sending a crafted stream of DTLS traffic to an affected device. A successful exploit could allow the attacker to cause the device to reload, resulting in a DoS condition.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/16/2026
The Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software for 3100 Series and 4200 Series devices are susceptible to a denial of service vulnerability within the Datagram Transport Layer Security implementation. This specific flaw resides in how these systems handle incoming DTLS messages, which are critical for establishing secure tunnels such as IPsec or SSL VPN connections. The root cause is identified as improper resource management during the processing phase of certain malformed or crafted DTLS packets. When an attacker sends a specifically constructed stream of DTLS traffic to the affected device, the software fails to correctly allocate or release system resources associated with these messages. This inefficiency leads to a gradual exhaustion of memory or CPU cycles dedicated to handling the connection state machine, ultimately destabilizing the firewall's operational integrity.
From a technical perspective, this vulnerability aligns with CWE-401, which describes missing release of memory after effective usage, and CWE-787, indicating out-of-bounds write access if the resource exhaustion leads to buffer corruption or stack overflow scenarios during processing. The attack vector is remote and requires no authentication, allowing any external actor on the network path to initiate the exploit. By flooding the device with crafted DTLS packets that trigger this improper handling routine, an attacker can force the firewall into a state where it cannot process legitimate traffic efficiently. In severe cases, as noted in vendor advisories, this resource exhaustion causes the device to reload unexpectedly. This automatic reboot is a protective mechanism triggered by critical system errors but results in a complete interruption of network security services during the downtime period.
The operational impact of exploiting this vulnerability is significant for organizations relying on these firewalls for perimeter defense and secure remote access. A successful exploitation leads to a denial of service condition, effectively taking the firewall offline or degrading its performance to the point where it cannot enforce policies or maintain encrypted tunnels. For critical infrastructure or enterprise networks, even brief periods of unfiltered traffic can expose internal assets to unauthorized access or malicious activity that would normally be blocked by the ASA or FTD software. The lack of authentication requirements for this attack makes it particularly dangerous in open internet-facing environments where DTLS services are exposed without strict source IP filtering or rate limiting controls.
To mitigate this risk, administrators should ensure that their Cisco Secure Firewall 3100 and 4200 Series devices are running the latest software versions provided by Cisco, which contain patches addressing these resource management flaws in the DTLS stack. Additionally, network segmentation strategies can be employed to restrict access to DTLS ports from untrusted networks until updates are applied. Implementing rate limiting on incoming UDP traffic destined for VPN services can also help mitigate the impact of such floods by dropping excessive packets before they trigger the vulnerable code paths. Monitoring system logs for unexpected reload events or high resource utilization spikes related to SSL/IPsec processes is recommended as an early detection mechanism for potential exploitation attempts in real-time environments.