Introduction
Remote attackers with valid VPN credentials can achieve root-level code execution on Cisco Secure Firewall ASA and FTD devices, potentially resulting in total compromise of critical network security infrastructure. This vulnerability affects the VPN web server component, which is commonly enabled in enterprise deployments for remote access and site-to-site VPN functionality.
Cisco Secure Firewall ASA (Adaptive Security Appliance) and FTD (Firepower Threat Defense) are widely used in enterprise and service provider networks to deliver firewall, VPN, and advanced threat protection services. Cisco is a dominant force in the network security industry, with millions of devices deployed globally across critical sectors including finance, healthcare, and government.
Technical Information
CVE-2025-20333 is a buffer overflow vulnerability (CWE-120) in the VPN web server of Cisco ASA and FTD software. The flaw is due to improper validation of 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 this input into an internal buffer without adequate bounds checking. This classic buffer overflow condition allows the attacker to overwrite adjacent memory, including critical system structures or function pointers. As a result, arbitrary code execution as root is possible. The vulnerability is only exploitable by authenticated VPN users and affects systems with VPN web server functionality enabled. No public code or exploit payloads are available in the referenced sources.
Affected Systems and Versions
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software
- Cisco Secure Firewall Threat Defense (FTD) Software
- Only systems with VPN web server functionality enabled are affected
- The vulnerability is accessible to any authenticated VPN user
- No specific version numbers or ranges are provided in the referenced materials
Vendor Security History
Cisco has a history of similar vulnerabilities in the ASA and FTD product lines, 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 The company typically issues advisories and patches promptly and provides tools like the Cisco Software Checker for exposure assessment. However, recurring input validation and buffer management flaws indicate ongoing challenges in secure development practices for these complex appliances.