CVE-2026-20103 in Secure Firewall Adaptive Security Appliance Software
Summary
by MITRE • 03/04/2026
A vulnerability in the Remote Access SSL VPN functionality of Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Secure Firewall Threat Defense (FTD) Software could allow an unauthenticated, remote attacker to exhaust device memory resulting in a denial of service (DoS) condition to new Remote Access SSL VPN connections. This does not affect the management interface, though it may become temporarily unresponsive. This vulnerability is due to trusting user input without validation. An attacker could exploit this vulnerability by sending crafted packets to the Remote Access SSL VPN server. A successful exploit could allow the attacker to cause the device web interface to stop responding, resulting in a DoS condition.
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Analysis
by VulDB Data Team • 05/02/2026
The vulnerability identified as CVE-2026-20103 represents a critical memory exhaustion flaw within Cisco's Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense software implementations. This weakness specifically targets the Remote Access SSL VPN functionality, creating a pathway for unauthenticated remote attackers to systematically deplete device resources through carefully crafted network packets. The flaw stems from insufficient input validation mechanisms that fail to properly sanitize or verify user-provided data before processing, allowing malicious payloads to exploit the system's memory management protocols. The vulnerability affects the core operational integrity of network security infrastructure, potentially disrupting legitimate user access to remote resources while maintaining the device's ability to process management interface requests, albeit with temporary unresponsiveness.
The technical exploitation of this vulnerability occurs through the manipulation of SSL VPN server protocols where attackers can send malformed or specially constructed packets that trigger memory allocation processes without proper bounds checking. This allows the attacker to consume available memory resources in a manner that prevents the system from allocating necessary resources for new SSL VPN connections, effectively creating a denial of service condition. The flaw operates at the application layer of the network stack, specifically targeting the SSL VPN processing engine within the firewall software, making it particularly dangerous for organizations that rely heavily on remote access capabilities. The vulnerability's impact extends beyond simple service disruption as it can render the entire SSL VPN service unavailable to legitimate users while potentially affecting the device's overall stability and responsiveness.
From an operational standpoint, this vulnerability presents significant risk to enterprise security infrastructure, particularly for organizations that depend on remote access capabilities for business continuity. The DoS condition affects new connection attempts rather than existing sessions, meaning that while established connections may remain functional, the system becomes unable to accommodate new remote access requests. This creates a cascading effect where legitimate users attempting to establish connections face service interruptions, potentially impacting productivity and business operations. The temporary unresponsiveness of the management interface adds another layer of complexity to incident response, as administrators may find it difficult to diagnose or remediate the issue while the system is under attack. Organizations with multiple firewall appliances may experience widespread disruption if the vulnerability affects their entire deployment.
Mitigation strategies for CVE-2026-20103 should prioritize immediate software updates from Cisco to address the root cause of input validation failures and memory handling issues. Network administrators should implement rate limiting and connection throttling measures to prevent rapid memory exhaustion attacks, while also monitoring for unusual traffic patterns that may indicate exploitation attempts. The implementation of intrusion detection systems with signature-based detection capabilities can help identify and block malicious packet sequences targeting this vulnerability. Additionally, organizations should consider implementing network segmentation to isolate SSL VPN services from critical infrastructure, reducing the potential impact of successful exploitation. The vulnerability aligns with CWE-129, which addresses improper validation of input length, and may be categorized under ATT&CK technique T1499.004 for network denial of service attacks. Security teams should also establish incident response procedures specifically addressing SSL VPN DoS conditions and maintain detailed logs of connection attempts to facilitate forensic analysis and threat hunting activities.