CVE-2017-6720 in Stackable Managed Switch
Summary
by MITRE
A vulnerability in the Secure Shell (SSH) subsystem of Cisco Small Business Managed Switches software could allow an authenticated, remote attacker to cause a reload of the affected switch, resulting in a denial of service (DoS) condition. The vulnerability is due to improper processing of SSH connections. An attacker could exploit this vulnerability by logging in to an affected switch via SSH and sending a malicious SSH message. This vulnerability affects the following Cisco products when SSH is enabled: Small Business 300 Series Managed Switches, Small Business 500 Series Stackable Managed Switches, 350 Series Managed Switches, 350X Series Stackable Managed Switches, 550X Series Stackable Managed Switches, ESW2 Series Advanced Switches. Cisco Bug IDs: CSCvb48377.
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Analysis
by VulDB Data Team • 01/13/2021
The vulnerability described in CVE-2017-6720 represents a critical denial of service flaw within Cisco Small Business Managed Switches that leverages the Secure Shell protocol as its attack vector. This weakness specifically targets the SSH subsystem implementation in various Cisco switch models, creating a scenario where authenticated remote attackers can trigger system reloads and consequently disrupt network operations. The vulnerability stems from inadequate handling of SSH connection processing, which allows maliciously crafted SSH messages to cause unexpected behavior in the affected network infrastructure. The impact extends across multiple product lines including Small Business 300 Series, 500 Series Stackable Managed Switches, 350 Series, 350X Series Stackable Managed Switches, 550X Series Stackable Managed Switches, and ESW2 Series Advanced Switches, all of which must have SSH enabled to be susceptible to exploitation.
The technical exploitation of this vulnerability requires an attacker to first establish a legitimate SSH session with the target switch, which then provides the attacker with the necessary authentication credentials to send malicious SSH messages. This process aligns with the attack pattern documented in the MITRE ATT&CK framework under the technique of privilege escalation and defense evasion, where attackers leverage legitimate administrative access to cause system instability. The improper processing of SSH connections manifests as a buffer overflow or memory corruption issue that leads to the switch's operating system crashing and subsequently reloading. This behavior directly violates the principle of least privilege and system integrity as defined by cybersecurity standards, since the legitimate SSH service becomes a vector for unauthorized disruption of network services. The vulnerability specifically relates to CWE-121, which describes heap-based buffer overflow conditions, and CWE-122, which covers buffer overflow vulnerabilities that occur when more data is written to a buffer than it can hold.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise network availability and business continuity for organizations relying on these managed switches. When switches undergo unexpected reloads, network traffic experiences temporary interruption as the devices reinitialize their routing tables and reestablish connections, creating cascading effects throughout the network infrastructure. Network administrators may experience difficulty in maintaining service levels as these devices require manual intervention to restore normal operations, and automated network monitoring systems may incorrectly flag these reloads as security incidents or hardware failures. The vulnerability affects enterprise networks where these switches serve as critical components in the network backbone, potentially causing widespread disruption to business operations and requiring immediate remediation efforts. Organizations utilizing these switches must consider the potential for prolonged service outages during maintenance windows when applying security patches, as the switch reload process itself creates temporary network disruption.
Mitigation strategies for CVE-2017-6720 should prioritize immediate implementation of Cisco's security advisory and patch management procedures, as the vulnerability requires active exploitation rather than passive scanning. Network administrators should disable SSH access on affected switches where possible and implement alternative management protocols such as HTTPS or SNMP with appropriate authentication controls. The implementation of network segmentation and access control measures can limit the scope of potential exploitation by restricting SSH access to only authorized administrative personnel. Regular security audits should verify that SSH services are properly configured with strong authentication mechanisms and that unnecessary services are disabled. Organizations should also implement network monitoring solutions that can detect unusual reload patterns and alert administrators to potential exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date security patches and conducting regular vulnerability assessments to prevent exploitation of known weaknesses in network infrastructure components. Additionally, implementing network access control lists and firewall rules to restrict SSH access to specific administrative IP ranges can significantly reduce the attack surface and prevent unauthorized access to these vulnerable network devices.