CVE-2018-15862 in xkbcommon
Summary
by MITRE
Unchecked NULL pointer usage in LookupModMask in xkbcomp/expr.c in xkbcommon before 0.8.2 could be used by local attackers to crash (NULL pointer dereference) the xkbcommon parser by supplying a crafted keymap file with invalid virtual modifiers.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15862 represents a critical null pointer dereference flaw within the xkbcommon library's xkbcomp component. This issue exists in the LookupModMask function located in xkbcomp/expr.c and affects versions prior to 0.8.2. The xkbcommon library serves as a fundamental component for handling keyboard layouts and key mappings across various linux desktop environments and applications, making this vulnerability particularly concerning for system security and stability.
The technical flaw stems from unchecked null pointer usage during the processing of keymap files that contain invalid virtual modifiers. When the xkbcomp parser encounters a crafted keymap file with malformed virtual modifier definitions, the LookupModMask function fails to properly validate pointer references before dereferencing them. This lack of input validation creates a condition where a null pointer is accessed, leading to an immediate system crash. The vulnerability specifically targets the parsing logic that handles virtual modifier mappings, which are essential components in defining complex keyboard behaviors and modifier key combinations.
From an operational impact perspective, this vulnerability provides local attackers with a reliable method to cause denial of service attacks against systems utilizing xkbcommon. The crash occurs during the keymap parsing phase, which means any application or service that processes keyboard configuration files could be affected, including desktop environments, display servers, and system configuration tools. The simplicity of exploitation makes this particularly dangerous as attackers need only provide a malformed keymap file to trigger the vulnerability, potentially affecting multiple system components that rely on xkbcommon for keyboard input handling.
The vulnerability aligns with CWE-476 which specifically addresses null pointer dereference issues, and represents a classic example of insufficient input validation in security-critical parsing functions. From an attack framework perspective, this vulnerability could be leveraged as part of a broader attack chain where an attacker first gains local access to a system, then uses this vulnerability to escalate the impact through service disruption or potentially as a stepping stone for further exploitation. The ATT&CK framework would classify this under privilege escalation and denial of service tactics, as it allows local adversaries to compromise system availability and potentially disrupt user sessions.
Mitigation strategies should focus on immediate patching of affected systems to upgrade to xkbcommon version 0.8.2 or later, where the null pointer validation has been implemented. System administrators should also implement monitoring for unusual keymap file processing activities and consider restricting write permissions to keyboard configuration directories. Additionally, applications that utilize xkbcommon should implement proper error handling and input validation to prevent cascading failures, while organizations should maintain updated threat intelligence on similar parsing vulnerabilities in input processing libraries to prevent similar issues from arising in other components of their software stack.