CVE-2013-1193 in Firewall Services Module
Summary
by MITRE
The Secure Shell (SSH) implementation on Cisco Adaptive Security Appliances (ASA) devices, and in Cisco Firewall Services Module (FWSM), does not properly terminate sessions, which allows remote attackers to cause a denial of service (SSH service outage) by repeatedly establishing SSH connections, aka Bug IDs CSCue63881, CSCuf51892, CSCue78671, and CSCug26937.
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Analysis
by VulDB Data Team • 04/29/2017
The vulnerability identified as CVE-2013-1193 affects Cisco Adaptive Security Appliances and Firewall Services Module devices, specifically targeting their Secure Shell implementation. This flaw represents a critical denial of service weakness that undermines the availability of essential network security services. The issue manifests when the SSH service fails to properly terminate sessions, creating a condition where malicious actors can exploit this behavior to disrupt legitimate network operations. The vulnerability impacts organizations that rely on these security appliances for remote management and administrative access, potentially exposing their infrastructure to operational disruptions.
The technical root cause of this vulnerability lies in improper session management within the SSH daemon implementation on affected Cisco devices. When remote attackers repeatedly establish SSH connections without proper session cleanup, the system accumulates inactive connections that consume available resources. This resource exhaustion eventually leads to the SSH service becoming unresponsive or completely unavailable. The flaw demonstrates characteristics consistent with CWE-400, which addresses Uncontrolled Resource Consumption, and aligns with ATT&CK technique T1499.1, specifically targeting the availability of services through resource exhaustion attacks. The improper session termination creates a persistent state where connection handles remain allocated even after the session should have ended, leading to progressive degradation of service availability.
The operational impact of this vulnerability extends beyond simple service disruption, potentially affecting critical network infrastructure management capabilities. Organizations utilizing these security appliances for remote administration face significant risk of operational downtime, particularly during peak maintenance periods or security incidents when remote access is most critical. The vulnerability can be exploited with relatively simple tools and techniques, making it accessible to threat actors with basic networking knowledge. This makes it particularly dangerous in environments where security appliances serve as primary access points for network management, as unauthorized users could effectively disable critical security controls. The attack requires minimal resources and can be executed continuously, making it difficult to distinguish from legitimate traffic patterns and complicating detection efforts.
Mitigation strategies for CVE-2013-1193 should focus on both immediate protective measures and long-term architectural improvements. Cisco recommends applying the relevant security patches and firmware updates that address the session management flaw directly. Network administrators should implement connection rate limiting and monitoring to detect abnormal SSH connection patterns that may indicate exploitation attempts. The implementation of intrusion detection systems with signature-based detection capabilities can help identify and block malicious connection sequences targeting this vulnerability. Additionally, organizations should consider implementing alternative access methods such as VPN-based remote access or privileged access management solutions to reduce reliance on direct SSH connections. Network segmentation and access control measures can further limit the potential impact of such attacks by containing them within specific network segments and preventing lateral movement. The vulnerability highlights the importance of proper resource management in network security appliances and demonstrates the need for robust session handling mechanisms that prevent resource exhaustion attacks. Organizations should also establish monitoring procedures that track SSH service availability and connection counts to provide early warning of potential exploitation attempts.