CVE-2011-0949 in IOS XR
Summary
by MITRE
Cisco IOS XR 3.6.x, 3.8.x before 3.8.3, and 3.9.x before 3.9.1 does not properly remove sshd_lock files from /tmp/, which allows remote attackers to cause a denial of service (disk consumption) by making many SSHv1 connections, aka Bug ID CSCtd64417.
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Analysis
by VulDB Data Team • 03/23/2017
Cisco IOS XR software versions 3.6.x through 3.8.2 and 3.9.x through 3.9.0 contain a critical flaw in their Secure Shell implementation that enables remote attackers to consume excessive disk space and potentially cause system denial of service. The vulnerability stems from improper handling of sshd_lock files within the /tmp/ directory during SSHv1 connection processing. When multiple SSHv1 connections are established simultaneously, the system fails to properly clean up temporary lock files, leading to accumulation in the /tmp/ filesystem. This behavior represents a classic resource exhaustion attack vector that directly impacts system availability and can result in complete service disruption. The flaw is categorized under CWE-362, which addresses race conditions and improper resource management in security-critical systems. From an operational security perspective, this vulnerability aligns with ATT&CK technique T1499.004, specifically targeting availability through resource consumption attacks. The impact extends beyond simple denial of service as it can affect network infrastructure reliability and potentially compromise the integrity of the routing platform. Attackers can exploit this vulnerability by establishing numerous concurrent SSHv1 connections, causing the /tmp/ filesystem to fill up with persistent lock files that are not automatically cleaned. This creates a cascading effect where legitimate SSH connections may fail due to insufficient disk space, and system administrators may face challenges in diagnosing the root cause of service degradation. The vulnerability affects Cisco IOS XR implementations across multiple release trains, indicating a fundamental flaw in the SSH daemon implementation that requires immediate attention. The lack of proper cleanup mechanisms for temporary files represents a significant oversight in the software's resource management and security design. Network operators using affected Cisco IOS XR versions face substantial risk of operational disruption, particularly in environments where SSH access is heavily utilized for network management and monitoring purposes. The vulnerability demonstrates the critical importance of proper temporary file handling and cleanup procedures in security-sensitive applications, especially those operating in mission-critical network infrastructure environments where availability is paramount. Organizations should prioritize immediate patching of affected systems to prevent exploitation and maintain network operational integrity. The flaw also highlights the need for comprehensive testing of resource management behaviors under high-concurrency scenarios to identify similar vulnerabilities in other network services and protocols.
The technical exploitation of this vulnerability requires minimal sophistication and can be executed remotely without authentication. Attackers simply need to establish multiple SSHv1 connections to the affected device, triggering the accumulation of lock files in /tmp/ which gradually consumes available disk space. This type of attack falls under the category of low-effort, high-impact denial of service vectors that can severely impact network operations and service availability. The vulnerability's persistence across multiple IOS XR release versions indicates a systemic issue in the codebase that requires thorough code review and remediation. Network administrators should implement monitoring solutions to detect unusual disk space consumption patterns and establish automated alerting mechanisms to identify potential exploitation attempts. The flaw also demonstrates the importance of proper input validation and resource cleanup in network protocol implementations, particularly when dealing with connection-oriented services that may experience high concurrent usage patterns. Organizations should conduct comprehensive vulnerability assessments across their entire network infrastructure to identify other potentially affected systems and implement appropriate security controls to mitigate similar risks. The incident underscores the critical need for robust security testing practices that include stress testing and resource exhaustion scenarios to ensure network infrastructure resilience against such attacks.