CVE-2017-12164 in GDM
Summary
by MITRE
A flaw was discovered in gdm 3.24.1 where gdm greeter was no longer setting the ran_once boolean during autologin. If autologin was enabled for a victim, an attacker could simply select 'login as another user' to unlock their screen.
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Analysis
by VulDB Data Team • 04/25/2023
The vulnerability identified as CVE-2017-12164 represents a critical security flaw in the GNOME Display Manager version 3.24.1 that fundamentally undermines the integrity of the autologin protection mechanism. This issue affects the gdm greeter component which is responsible for managing user authentication and session initiation within the GNOME desktop environment. The flaw manifests in the improper handling of the ran_once boolean flag, which serves as a critical security control during the authentication process. When autologin is configured for a user account, the system should ensure that the authentication sequence executes only once and that subsequent login attempts are properly restricted. The failure to correctly manage this boolean flag creates a persistent security weakness that directly impacts the session management and access control mechanisms of the display manager.
The technical exploitation of this vulnerability occurs through a specific user interaction pattern that bypasses the intended security controls. When a victim system has autologin enabled, an attacker who gains physical access to the machine can simply select the 'login as another user' option from the greeter interface to gain access to the locked screen. This action exploits the improper state management where the system fails to recognize that the initial authentication has already been completed and validated. The vulnerability essentially allows an attacker to circumvent the authentication flow that should prevent unauthorized access to accounts that have been configured for automatic login. This flaw operates at the interface level where user choices are processed without proper validation of the authentication state, creating an attack vector that requires minimal technical expertise to exploit.
The operational impact of CVE-2017-12164 extends beyond simple unauthorized access to encompass broader implications for system security and user privacy. Organizations relying on GNOME desktop environments with autologin configured face significant risks as this vulnerability can be exploited by anyone with physical access to the affected systems. The attack does not require network connectivity or advanced technical knowledge, making it particularly dangerous in environments where physical security may be compromised. This vulnerability directly violates the principle of least privilege and can lead to data breaches, unauthorized system modifications, and potential escalation to administrative privileges. The flaw affects the core authentication mechanism of the display manager, which serves as the first line of defense for desktop systems, rendering the configured autologin protection ineffective against determined attackers.
This vulnerability maps to CWE-284 Access Control Bypass, which specifically addresses situations where access controls are improperly enforced, allowing unauthorized users to access resources or perform actions they should not be permitted to execute. The flaw also aligns with ATT&CK technique T1078 Valid Accounts, as it exploits legitimate user accounts through the bypass of authentication mechanisms that should protect them. The vulnerability demonstrates a failure in proper state management within the authentication flow, where the system does not correctly maintain the authentication state after the initial login process has been completed. Organizations should implement immediate mitigations including disabling autologin for sensitive systems, ensuring proper physical security controls, and applying the relevant patches that address the boolean flag management issue in gdm components. The remediation process requires updating to patched versions of GNOME Display Manager where the ran_once flag is properly maintained throughout the authentication lifecycle, effectively restoring the intended security controls for user session management and access protection.