Introduction
Remote attackers with valid VPN credentials can execute code as root on Cisco Secure Firewall ASA and FTD devices, which could mean total compromise of critical network security infrastructure. The flaw is in the VPN web server component, which enterprises commonly enable for remote access and site-to-site VPN.
Cisco Secure Firewall ASA (Adaptive Security Appliance) and FTD (Firepower Threat Defense) are widely used in enterprise and service provider networks for firewall, VPN, and advanced threat protection. Cisco dominates the network security industry, with millions of devices deployed worldwide in finance, healthcare, government, and other critical sectors.
Technical Information
CVE-2025-20333 is a buffer overflow vulnerability (CWE-120) in the VPN web server of Cisco ASA and FTD software. The server does not properly validate user-supplied input in HTTP(S) requests. When an authenticated VPN user sends a crafted HTTP request with oversized or malformed data, the server copies that input into an internal buffer without adequate bounds checking. This is a classic buffer overflow. The attacker can overwrite adjacent memory, including critical system structures or function pointers, and from there execute arbitrary code as root. Only authenticated VPN users can exploit the flaw, and only on systems with the VPN web server enabled. The referenced sources include no public exploit code or payloads.
Affected Systems and Versions
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
- Cisco Secure Firewall Threat Defense (FTD) Software
- Only systems with the VPN web server enabled are affected
- Any authenticated VPN user can reach the vulnerable code
- The referenced materials give no specific version numbers or ranges
Vendor Security History
The ASA and FTD product lines have had similar vulnerabilities before, including:
- CVE-2025-20243: Buffer overflow in VPN web services
- CVE-2025-20133: Input validation flaw in Remote Access SSL VPN
- CVE-2025-20263: Buffer overflow in web services interface
Cisco typically issues advisories and patches promptly, and it provides tools such as the Cisco Software Checker so customers can assess their exposure. The repeated input validation and buffer management flaws, though, point to ongoing problems with secure development practices for these complex appliances.
