CVE-2006-1335 in Screensaver
Summary
by MITRE
gnome screensaver before 2.14, when running on an X server with AllowDeactivateGrabs and AllowClosedownGrabs enabled, allows attackers with physical access to cause the screensaver to crash and access the session via the Ctl+Alt+Keypad-Multiply keyboard sequence, which removes the grab from gnome.
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Analysis
by VulDB Data Team • 01/11/2019
The vulnerability described in CVE-2006-1335 represents a significant security flaw in the gnome screensaver implementation prior to version 2.14. This issue specifically affects systems running the gnome desktop environment where the X server has certain dangerous configuration options enabled. The vulnerability arises from the insecure handling of keyboard input sequences within the screensaver component, creating an unexpected attack vector that leverages physical access to compromise system security. The flaw exists in the way the screensaver manages input grabs and keyboard event processing, particularly when dealing with specific modifier key combinations that should normally be restricted during screensaver operation.
The technical exploitation of this vulnerability relies on the specific X server configuration parameters AllowDeactivateGrabs and AllowClosedownGrabs being enabled. These parameters, when active, allow external applications to manipulate grab states on the X server, which in turn creates a window of opportunity for malicious actors. The attack vector specifically targets the Ctl+Alt+Keypad-Multiply keyboard sequence, which when pressed during screensaver operation causes the gnome screensaver to crash and subsequently removes the grab that was preventing access to the session. This sequence effectively bypasses the screensaver's security mechanisms and allows immediate access to the desktop session without proper authentication, as the grab removal enables normal keyboard input processing that would otherwise be blocked.
The operational impact of this vulnerability is particularly concerning as it requires only physical access to the system to exploit, making it especially dangerous in environments where unauthorized physical access is possible. The vulnerability essentially provides an attacker with a straightforward method to bypass session locking, which could lead to unauthorized access to sensitive data, system compromise, or further lateral movement within a network. This type of vulnerability falls under the category of privilege escalation through session hijacking, where an attacker can effectively bypass the security controls designed to protect user sessions when they are idle or locked. The vulnerability demonstrates poor input validation and improper handling of X server grab states, creating a direct path to session access that should have been protected by the screensaver mechanism.
The security implications extend beyond simple session access, as this vulnerability can be exploited to gain unauthorized access to applications and data that were protected by the screensaver's lock mechanism. Systems running vulnerable versions of gnome screensaver are particularly at risk in shared or public environments where physical access might be obtained by unauthorized parties. The vulnerability also highlights the importance of proper X server configuration and the dangers of enabling potentially dangerous options without understanding their security implications. This issue is related to CWE-254, which addresses security weaknesses in input handling and improper access control, and can be mapped to ATT&CK technique T1547.001, which involves hijacking system processes to maintain persistence or gain access to protected resources. The vulnerability represents a classic example of how seemingly benign X server configuration options can create security holes when combined with insecure application design, emphasizing the need for comprehensive security reviews of both system-level configurations and application-level security controls. Organizations should ensure that all systems running gnome desktop environments are updated to versions that properly address this vulnerability and that X server configurations are reviewed to minimize the exposure of dangerous options.