CVE-2019-1836 in NX-OS
Summary
by MITRE
A vulnerability in the system shell for Cisco Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) mode could allow an authenticated, local attacker to use symbolic links to overwrite system files. These system files may be sensitive and should not be overwritable by non-root users. The attacker would need valid device credentials. The vulnerability is due to incorrect symbolic link verification of directory paths when they are used in the system shell. An attacker could exploit this vulnerability by authenticating to the device and providing crafted user input to specific symbolic link CLI commands. Successful exploitation could allow the attacker to overwrite system files that should be restricted. This vulnerability has been fixed in software version 14.1(1i).
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Analysis
by VulDB Data Team • 09/12/2023
The vulnerability identified as CVE-2019-1836 affects Cisco Nexus 9000 Series Fabric Switches operating in Application Centric Infrastructure (ACI) mode, representing a critical security flaw in the system shell implementation. This weakness stems from inadequate verification mechanisms for symbolic link directory paths within the command-line interface, creating a pathway for authenticated local attackers to escalate privileges and compromise system integrity. The vulnerability specifically targets the improper handling of symbolic link operations during shell execution, where the system fails to properly validate directory paths before allowing symbolic link creation or manipulation. The flaw enables attackers with valid device credentials to exploit the system through crafted CLI commands that leverage symbolic link manipulation to overwrite critical system files. This represents a significant bypass of the principle of least privilege, as the system incorrectly permits non-root users to modify files that should remain restricted and protected. The vulnerability exists in the underlying shell execution engine of the Cisco Nexus 9000 platform, making it particularly dangerous given that the device operates in a network infrastructure environment where such access could lead to widespread compromise.
The technical exploitation of this vulnerability requires an authenticated attacker with valid device credentials, which significantly reduces the attack surface compared to remote exploits. However, the impact remains severe as the attacker can leverage the symbolic link verification flaw to overwrite sensitive system files that should be protected from modification by regular users. The vulnerability specifically manifests when the system shell processes symbolic link commands without proper validation of the target directory paths, allowing attackers to craft inputs that traverse directory structures and overwrite critical system components. This issue falls under the CWE-367 weakness category, which addresses Time-of-Check to Time-of-Use (TOCTOU) vulnerabilities, where the system's validation occurs at a different time than when the operation is executed. The exploitation process involves constructing specific symbolic link commands that leverage the shell's insufficient path validation to gain write access to restricted files, potentially leading to privilege escalation or system compromise. The vulnerability's impact extends beyond simple file modification as it undermines the fundamental security model of the device by allowing unauthorized modification of system-critical components.
The operational impact of CVE-2019-1836 presents a substantial risk to network infrastructure security, particularly within enterprise environments where Cisco Nexus 9000 switches serve as core network fabric components. Successful exploitation could enable attackers to modify system files that control the device's operation, potentially leading to complete system compromise, service disruption, or data exfiltration. The vulnerability's presence in ACI mode is particularly concerning as it affects the application-centric networking framework that many enterprises rely upon for network segmentation and policy enforcement. Network administrators face the challenge of identifying and mitigating this vulnerability without disrupting critical network operations, as the attack requires local access to the device. The vulnerability's exploitation could result in persistent backdoors or the corruption of system binaries, leading to device instability or complete failure. Organizations using Cisco Nexus 9000 switches in ACI mode must consider the potential for this vulnerability to be leveraged as part of broader attack campaigns targeting network infrastructure, particularly in environments where internal network access is not adequately restricted. The attack vector's requirement for valid credentials means that insider threats or compromised accounts pose a significant risk, making proper access control and credential management critical defensive measures.
Mitigation of CVE-2019-1836 requires immediate implementation of the vendor-provided software patch version 14.1(1i), which addresses the symbolic link verification flaw in the system shell. Organizations should conduct comprehensive vulnerability assessments to identify all affected Cisco Nexus 9000 switches operating in ACI mode and ensure proper patch deployment across their network infrastructure. Network administrators should implement strict access controls and credential management practices, including regular credential rotation and monitoring of device access logs for suspicious activities. The implementation of network segmentation and least privilege principles can help limit the potential impact of successful exploitation by reducing the attack surface for local access. Security monitoring should include detection of unusual symbolic link creation patterns and file modification activities on critical system components. Organizations should also consider implementing device hardening measures, such as disabling unnecessary services and restricting shell access to only authorized personnel. The vulnerability's resolution through the 14.1(1i) software update demonstrates the importance of maintaining current security patches and following vendor advisories for network infrastructure devices. Regular security assessments and vulnerability management processes should be implemented to identify similar weaknesses in other network components and ensure comprehensive protection against time-of-check to time-of-use vulnerabilities. The ATT&CK framework's T1078 technique for Valid Accounts and T1566 for Phishing can be relevant in understanding how this vulnerability might be exploited in broader attack scenarios, particularly when considering compromised credentials as the initial access vector.