CVE-2018-20839 in systemd
Summary
by MITRE
systemd 242 changes the VT1 mode upon a logout, which allows attackers to read cleartext passwords in certain circumstances, such as watching a shutdown, or using Ctrl-Alt-F1 and Ctrl-Alt-F2. This occurs because the KDGKBMODE (aka current keyboard mode) check is mishandled.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 10/24/2024
The vulnerability identified as CVE-2018-20839 affects systemd version 242 and represents a significant security flaw in terminal handling during system shutdown processes. This issue stems from improper management of virtual terminal (VT) modes when users log out of the system, creating an unintended information disclosure channel that could expose cleartext passwords to unauthorized parties. The vulnerability specifically manifests when systemd alters the VT1 mode during logout operations, potentially allowing attackers to capture sensitive authentication data through careful observation of system shutdown sequences.
The technical root cause of this vulnerability lies in the mishandling of the KDGKBMODE system call, which is responsible for determining the current keyboard mode of the terminal. When systemd processes logout events, it fails to properly maintain the terminal state, leading to a situation where keyboard input mode transitions occur inappropriately. This malfunction creates a window of opportunity where cleartext passwords entered during shutdown procedures can be captured by attackers who observe the system behavior through keyboard input monitoring or by accessing the virtual terminal directly. The vulnerability is particularly concerning because it can be exploited during routine system shutdowns when users might be entering passwords or other sensitive information.
The operational impact of this vulnerability extends beyond simple password exposure, as it represents a fundamental breakdown in terminal security during system transitions. Attackers can exploit this weakness by monitoring virtual terminals, particularly VT1 and VT2, when users perform shutdown operations or when system administrators access alternative terminals using Ctrl-Alt-F1 and Ctrl-Alt-F2 key combinations. The vulnerability is especially dangerous in multi-user environments where system administrators might be logging out or performing maintenance operations, as it could allow unauthorized individuals to capture authentication credentials. This type of information disclosure aligns with CWE-200, which addresses "Information Exposure" vulnerabilities, and represents a specific instance where system state management fails to protect sensitive data.
The exploitation of this vulnerability requires minimal technical expertise and can be accomplished through passive observation of system shutdown procedures or by accessing virtual terminals during system transitions. Security researchers have identified that this issue affects systems running systemd version 242 and potentially earlier versions, making it a widespread concern across various Linux distributions that incorporate this version of the system initialization framework. The vulnerability demonstrates poor adherence to security best practices in terminal state management and highlights the importance of proper system call handling during critical operations. Organizations should implement immediate mitigations including updating to systemd versions that address this vulnerability, as well as monitoring virtual terminal access during shutdown procedures to prevent unauthorized observation of sensitive input.
This vulnerability also connects to broader security concepts within the ATT&CK framework, specifically relating to credential access techniques where adversaries may exploit system configuration weaknesses to capture authentication data. The flaw represents a failure in privilege separation and proper terminal state management that could be leveraged in more sophisticated attacks. System administrators should consider implementing additional monitoring controls and ensuring that terminal sessions are properly secured during shutdown operations to prevent unauthorized access to sensitive input data. The remediation process involves updating systemd to versions that properly handle keyboard mode transitions and implementing proper terminal state management protocols to prevent similar issues from occurring in other system components.