CVE-2025-20133: 
Cisco Adaptive Security Appliance (ASA) vulnerability analysis and mitigation

A vulnerability (CVE-2025-20133) was discovered in the management and VPN web servers of the Remote Access SSL VPN feature of Cisco Secure Firewall ASA Software and Secure FTD Software. The vulnerability was identified on August 14, 2025, with a CVSS Base Score of 8.6 (High). This security flaw affects Cisco Secure Firewall ASA Software and Secure FTD Software that have SSL listen sockets enabled (Cisco Advisory).

The vulnerability stems from ineffective validation of user-supplied input during the Remote Access SSL VPN authentication process. It has been assigned a CVSS vector of CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H, indicating network accessibility, low attack complexity, and no required privileges or user interaction. The vulnerability is associated with CWE-401 (Missing Release of Memory after Effective Lifetime) (NVD, Cisco Advisory).

A successful exploitation of this vulnerability could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition where the device stops responding to Remote Access SSL VPN authentication requests. This could significantly impact the availability of VPN services for legitimate users (Cisco Advisory).

Cisco has released software updates that address this vulnerability. There are no workarounds available. Customers with service contracts can obtain security fixes through their usual update channels. Those without service contracts should contact the Cisco Technical Assistance Center (TAC) (Cisco Advisory).

The vulnerability was released as part of Cisco's August 2025 Semiannual Cisco Secure Firewall ASA, Secure FMC, and Secure FTD Software Security Advisory Bundled Publication, which included multiple high-severity vulnerabilities (Hacker News).