CVE-2015-7496 in Display Manager
Summary
by MITRE
GNOME Display Manager (gdm) before 3.18.2 allows physically proximate attackers to bypass the lock screen by holding the Escape key.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 06/27/2022
The vulnerability identified as CVE-2015-7496 affects the GNOME Display Manager (gdm) version 3.18.2 and earlier, presenting a significant security risk through a design flaw in the lock screen authentication mechanism. This issue specifically impacts systems running GNOME desktop environments where gdm serves as the primary display manager responsible for handling user authentication and screen locking functionality. The vulnerability arises from insufficient input handling during the lock screen state, creating an exploitable condition that allows unauthorized access to locked systems.
The technical flaw resides in the improper processing of keyboard input events while the system is in a locked state. When a user locks their screen through gdm, the system should enforce strict authentication controls and prevent any bypass mechanisms from being triggered by physical input. However, the vulnerability allows attackers positioned physically near the device to hold down the escape key, which triggers an unintended code path that bypasses the normal authentication sequence. This occurs because the system fails to properly validate or filter keyboard inputs during the lock screen transition period, particularly when the escape key combination is processed in a manner that circumvents the established security controls.
The operational impact of this vulnerability extends beyond simple unauthorized access to potentially compromise entire system security postures. An attacker with physical proximity to a locked workstation can gain immediate access to the system without providing valid credentials, effectively neutralizing the security benefits of screen locking. This presents particular risk in environments where sensitive data is processed, such as corporate offices, government facilities, or financial institutions where physical security controls are assumed to be sufficient. The vulnerability essentially creates a backdoor that operates without requiring network access, authentication credentials, or complex exploitation techniques, making it particularly dangerous in environments where physical access controls are not strictly enforced.
This vulnerability aligns with CWE-284, which describes improper access control, and represents a clear violation of the principle of least privilege in system security design. The flaw demonstrates poor input validation and event handling mechanisms that should have been implemented to prevent unauthorized system access. From an ATT&CK framework perspective, this vulnerability maps to techniques involving privilege escalation and credential access, specifically targeting the initial access phase through physical proximity attacks. The vulnerability also relates to T1078, which covers valid accounts, as it allows unauthorized access to systems through bypassing the authentication mechanism rather than through credential theft or brute force attacks.
Mitigation strategies for this vulnerability primarily involve updating to gdm version 3.18.2 or later, which contains the necessary patches to properly handle keyboard input during screen lock transitions. System administrators should also implement additional physical security measures such as securing workstations in locked areas, implementing automatic screen locking after periods of inactivity, and ensuring that users are educated about the risks of leaving systems unattended. Organizations should consider implementing additional monitoring solutions that can detect unusual keyboard input patterns or unauthorized access attempts. The patch for this vulnerability specifically addresses the input validation issue by ensuring that escape key combinations during lock screen states are properly filtered and do not trigger bypass mechanisms, thereby restoring the intended security controls for user authentication and system protection.