Introduction
Unexpected device reloads can instantly disrupt remote access for an entire workforce, halting business operations and exposing organizations to cascading failures. CVE-2025-20244 targets the Remote Access SSL VPN service in Cisco Secure Firewall Adaptive Security Appliance (ASA) and Cisco Secure Firewall Threat Defense (FTD) software, allowing an authenticated VPN user to trigger a denial of service condition by sending a crafted HTTP request.
Cisco ASA and FTD are core components in enterprise network security, widely deployed to provide firewall, VPN, and threat defense capabilities. Cisco is a global leader in networking and security, with its firewall and VPN products securing critical infrastructure for businesses, governments, and service providers worldwide.
Technical Information
CVE-2025-20244 stems from incomplete error checking when parsing HTTP header field values in the Remote Access SSL VPN service. When an authenticated VPN user sends a specially crafted HTTP request containing malformed header data, the device's error handling routines do not properly validate the input. This allows the malformed request to reach code paths that are not equipped to handle such data, resulting in an unexpected reload of the entire device.
The vulnerability is classified under CWE-1287 (Improper Validation of Specified Type of Input). The root cause is insufficient validation logic in the HTTP header parsing functions. The attack requires valid VPN credentials and access to the SSL VPN service. There are no public code snippets or proof of concept details available for this vulnerability.
The impact is a full device reload, causing a denial of service for all users and services relying on the affected Cisco ASA or FTD device. This includes firewall filtering, site-to-site VPNs, and other security functions. The reload process can take several minutes, during which network security and connectivity are disrupted.
Affected Systems and Versions
- Cisco Secure Firewall Adaptive Security Appliance (ASA) Software: Remote Access SSL VPN service
- Cisco Secure Firewall Threat Defense (FTD) Software: Remote Access SSL VPN service
Specific affected version numbers are not provided in the advisory. Organizations should refer to the official Cisco advisory for the latest affected and fixed version information:
Vendor Security History
Cisco has a documented history of critical vulnerabilities in its ASA and FTD product lines, especially in VPN and web services components. Notable recent issues include:
- CVE-2023-20269: Zero-day in ASA/FTD SSL VPN actively exploited by ransomware groups (Arctic Wolf, Tenable)
- CVE-2024-20481: Remote Access VPN DoS in ASA/FTD (The Hacker News)
Cisco typically responds rapidly with advisories and patches, but recurring input validation flaws in VPN and web services indicate persistent architectural challenges.