CVE-2025-20251: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

A vulnerability (CVE-2025-20251) was discovered in the Remote Access SSL VPN service for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software. The vulnerability was disclosed on August 14, 2025, and affects the VPN web server component of these systems. This security flaw has been assigned a CVSS base score of 8.5 (High) and is tracked as CWE-1287 (Cisco Advisory, NVD).

The vulnerability stems from insufficient input validation when processing HTTP requests in the Remote Access SSL VPN service. The flaw is classified under CWE-1287 (Improper Validation of Specified Type of Input). It has received a CVSS v3.1 base score of 8.5 (High) with the vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:L/A:H, indicating network accessibility, low attack complexity, and required low privileges for exploitation (Cisco Advisory).

A successful exploitation of this vulnerability could allow an attacker to create or delete arbitrary files on the underlying operating system. If critical system files are manipulated, it could result in new Remote Access SSL VPN sessions being denied and existing sessions being dropped, causing a denial of service (DoS) condition. The affected device would require a manual reboot to recover from such an attack (Cisco Advisory).

Cisco has released software updates that address this vulnerability. There are no workarounds available for this vulnerability. Customers with service contracts can obtain security fixes through their usual update channels. Organizations are advised to upgrade to the fixed software version as soon as possible (Cisco Advisory).