CVE-2019-1859 in Small Business Switches
Summary
by MITRE
A vulnerability in the Secure Shell (SSH) authentication process of Cisco Small Business Switches software could allow an attacker to bypass client-side certificate authentication and revert to password authentication. The vulnerability exists because OpenSSH mishandles the authentication process. An attacker could exploit this vulnerability by attempting to connect to the device via SSH. A successful exploit could allow the attacker to access the configuration as an administrative user if the default credentials are not changed. There are no workarounds available; however, if client-side certificate authentication is enabled, disable it and use strong password authentication. Client-side certificate authentication is disabled by default.
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Analysis
by VulDB Data Team • 09/12/2023
The vulnerability identified as CVE-2019-1859 represents a critical security flaw in Cisco Small Business Switches software that directly impacts the Secure Shell authentication mechanism. This weakness stems from improper handling of the authentication process within OpenSSH implementation, creating a pathway for unauthorized access that bypasses the intended security controls. The flaw specifically affects the transition between different authentication methods, allowing attackers to circumvent client-side certificate authentication and fall back to password-based authentication. This vulnerability is particularly concerning because it undermines the layered security approach that organizations typically implement, where certificate-based authentication serves as a stronger alternative to password authentication. The vulnerability exists in the core authentication logic of the SSH service, making it a fundamental weakness in the device's security architecture.
The technical exploitation of this vulnerability occurs during the SSH connection establishment process when the switch fails to properly validate authentication methods. Attackers can leverage this flaw by initiating an SSH connection to the affected device, triggering the flawed authentication handling that allows them to bypass the certificate verification process. The vulnerability's impact is amplified because it operates at the protocol level rather than being a simple configuration issue, meaning that even properly configured certificate authentication can be circumvented. This type of flaw aligns with CWE-284, which addresses improper access control in authentication mechanisms, and specifically demonstrates how weaknesses in authentication logic can lead to privilege escalation and unauthorized administrative access. The vulnerability's exploitation does not require specialized tools or complex attack vectors, making it accessible to threat actors with basic network access and knowledge of SSH protocols.
The operational impact of CVE-2019-1859 extends beyond simple unauthorized access, as it fundamentally compromises the integrity of the device's security posture. When default credentials remain unchanged, attackers can gain administrative access to the switch configuration, potentially leading to complete network compromise. This vulnerability enables attackers to modify network configurations, redirect traffic, or establish persistence mechanisms within the network infrastructure. The security implications are particularly severe for small business environments where network administrators may not be fully aware of the importance of changing default credentials or may not properly configure certificate-based authentication. Organizations relying on these switches for network security may find their perimeter defenses weakened, as the vulnerability allows attackers to bypass the additional security layer that certificate authentication provides. The lack of workarounds for this vulnerability means that administrators must either disable the problematic authentication method or implement immediate mitigations.
Mitigation strategies for CVE-2019-1859 require immediate action from network administrators to address the authentication bypass vulnerability. The primary recommended approach involves disabling client-side certificate authentication if it is not essential for operations, as this authentication method is disabled by default in the affected switches. Organizations should implement strong password policies and ensure that default credentials are changed immediately upon device deployment. The ATT&CK framework categorizes this vulnerability under privilege escalation and credential access techniques, emphasizing the need for network segmentation and monitoring of authentication attempts. Additionally, administrators should consider implementing multi-factor authentication mechanisms and regular security audits to detect unauthorized access attempts. The vulnerability also highlights the importance of keeping network device firmware updated, as this flaw represents a known issue that has been addressed in subsequent software releases. Network monitoring should include detection of unusual authentication patterns that might indicate exploitation attempts, particularly around the transition between different authentication methods.