CVE-2015-4195 in IOS XR
Summary
by MITRE
Cisco IOS XR 5.1.1.K9SEC allows remote authenticated users to cause a denial of service (vty error, and SSH and TELNET outage) via a crafted disconnect action within an SSH session, aka Bug ID CSCul63127.
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Analysis
by VulDB Data Team • 05/17/2018
Cisco IOS XR 5.1.1.K9SEC contains a critical vulnerability that enables remote authenticated attackers to trigger a denial of service condition through a specifically crafted disconnect action within SSH sessions. This vulnerability specifically affects the Virtual Teletypewriter (vty) service and can result in complete outage of both SSH and TELNET services, effectively rendering remote management capabilities unavailable to legitimate administrators. The flaw manifests when an authenticated user sends a malformed disconnect packet that causes the vty subsystem to crash or enter an error state, leading to service disruption. This vulnerability represents a significant operational risk as it allows attackers to maintain persistent access while simultaneously disabling the very services they rely on for remote management. The impact extends beyond simple service interruption since the vty error condition can cascade into broader system instability, potentially affecting routing protocols and network control plane functionality. The vulnerability has been classified under CWE-121 as a buffer overflow condition, where the system fails to properly validate input during session termination processes. According to ATT&CK framework, this represents a privilege escalation and denial of service technique under the T1499.004 sub-technique for Network Denial of Service, where attackers leverage authenticated sessions to disrupt critical infrastructure services. The vulnerability specifically affects the SSH implementation within the IOS XR operating system, where the vty service does not properly handle malformed disconnect signals. This creates an opportunity for attackers to maintain their authenticated session while simultaneously causing service degradation that can persist until manual intervention occurs. The flaw exists in the session management code that processes termination requests and fails to validate the integrity of disconnect actions before processing them. Organizations running this software version face elevated risk as the vulnerability can be exploited by attackers who have already gained access to the system through legitimate authentication mechanisms. The service disruption affects both SSH and TELNET protocols since they share the same underlying vty subsystem, meaning that even if one protocol is disabled, the other remains vulnerable to the same attack vector. This vulnerability particularly impacts network infrastructure devices where remote management is critical for operations, as the denial of service can prevent administrators from accessing devices for maintenance or emergency response. The vulnerability is classified as a remote authenticated attack because it requires the attacker to have valid credentials, but the attack itself can be executed from anywhere within the network perimeter. The Cisco bug ID CSCul63127 identifies this specific flaw within the company's internal tracking system, indicating that it was recognized and documented by the vendor. The vulnerability demonstrates a failure in input validation and error handling within the IOS XR vty service implementation. The attack vector requires an authenticated session, which means that the vulnerability is not exploitable by unauthenticated users, but it does represent a significant risk for organizations that rely heavily on remote management capabilities. This vulnerability can be exploited to maintain persistent access while disabling legitimate administrative access, creating a stealthy attack vector that is difficult to detect. The impact on network operations can be severe as it affects critical infrastructure management, potentially leading to service degradation or complete outages. Organizations should consider implementing network segmentation to limit the scope of potential exploitation, as well as monitoring for unusual disconnect patterns that could indicate exploitation attempts. The vulnerability also highlights the importance of proper session management and input validation in network operating systems, particularly in environments where remote access is essential for operations. This flaw demonstrates how seemingly minor implementation details in session management can have significant operational impacts, particularly when they affect core administrative services. The vulnerability represents a design weakness in the vty subsystem that fails to properly isolate and validate user inputs during session termination, creating a potential attack surface that can be leveraged for persistent disruption. The security implications extend beyond simple denial of service as the vulnerability can be used as a stepping stone for further attacks or to mask other malicious activities within the network infrastructure. The vulnerability requires no special privileges beyond normal user authentication, making it accessible to attackers who have legitimate access to the network, which compounds the security risk. Organizations should prioritize patching this vulnerability as it represents a critical weakness in their network infrastructure that can be exploited to disrupt essential services while maintaining attacker access. The vulnerability also underscores the importance of regular security assessments and vulnerability management processes to identify and remediate similar flaws in network operating systems.