CVE-2011-2187 in xscreensaver
Summary
by MITRE
xscreensaver before 5.14 crashes during activation and leaves the screen unlocked when in Blank Only Mode and when DPMS is disabled, which allows local attackers to access resources without authentication.
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Analysis
by VulDB Data Team • 11/28/2019
The vulnerability identified as CVE-2011-2187 affects xscreensaver versions prior to 5.14, specifically exposing a critical flaw in the screen locking mechanism that fundamentally undermines system security. This issue manifests when the screensaver attempts to activate under specific conditions, creating a dangerous window of opportunity for unauthorized access. The vulnerability operates within the context of desktop security systems where visual screen locking is expected to provide authentication protection, yet fails to deliver this essential security function. The flaw represents a direct violation of security principles that require proper access control mechanisms to prevent unauthorized system interaction.
The technical implementation of this vulnerability stems from improper error handling during the screensaver activation sequence when operating in Blank Only Mode with DPMS (Display Power Management Signaling) disabled. During normal operation, xscreensaver should transition the display to a locked state that requires authentication to unlock, but the software fails to properly manage this transition process. When DPMS is disabled, the screensaver cannot properly coordinate with the display subsystem to enforce the locking mechanism, resulting in a scenario where the screen appears to be in a locked state while remaining fully accessible to local users. This condition creates a false sense of security that can be exploited by malicious actors within the local network or physical environment.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass broader security implications for systems relying on visual screen locking as a primary security control. Local attackers can exploit this condition to gain access to sensitive information, execute unauthorized commands, or perform other malicious activities without providing authentication credentials. The vulnerability is particularly concerning in multi-user environments where different users share the same physical system or when systems are located in areas where unauthorized physical access might occur. Security professionals should recognize that this flaw undermines the fundamental security model of screen locking, which is typically considered a baseline security control for protecting against casual unauthorized access.
This vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and demonstrates how inadequate error handling can create security weaknesses in authentication systems. The flaw also corresponds to ATT&CK technique T1077, which involves the exploitation of vulnerabilities to gain access to systems, particularly in the context of local privilege escalation. Organizations implementing xscreensaver as part of their desktop security strategy must understand that this vulnerability can be exploited by attackers with local access to perform unauthorized activities without the need for sophisticated attack vectors. The vulnerability essentially creates a backdoor that allows access to the system while maintaining the appearance of normal security operations.
The recommended mitigation approach involves upgrading to xscreensaver version 5.14 or later, which contains the necessary patches to properly handle the screen locking sequence under the affected conditions. System administrators should also consider implementing additional security controls such as automatic screen locking policies, enhanced monitoring of screen locking events, and regular security audits to detect potential exploitation attempts. Organizations may need to temporarily disable Blank Only Mode or ensure that DPMS is properly enabled as interim measures while implementing the permanent fix. The vulnerability underscores the importance of proper error handling and state management in security-critical applications, particularly those that manage access control and authentication mechanisms.