CVE-2018-15854 in xkbcommon
Summary
by MITRE
Unchecked NULL pointer usage in xkbcommon before 0.8.1 could be used by local attackers to crash (NULL pointer dereference) the xkbcommon parser by supplying a crafted keymap file, because geometry tokens were desupported incorrectly.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15854 represents a critical null pointer dereference flaw within the xkbcommon library version 0.8.1 and earlier. This library serves as a fundamental component for handling keyboard mappings in Linux environments, particularly within the X Window System and Wayland compositor frameworks. The issue arises from improper handling of geometry tokens during the parsing of keymap files, creating a scenario where local attackers can deliberately craft malicious input files that trigger system crashes. The vulnerability specifically manifests when the parser encounters certain geometry token structures that were previously deprecated but not correctly removed from the processing logic, leading to a situation where the code attempts to dereference a null pointer reference. This flaw exists at the intersection of software robustness and input validation, where the library fails to properly validate or handle edge cases in keyboard configuration data.
The technical exploitation of this vulnerability occurs through the manipulation of keymap files that contain malformed geometry tokens, which are part of the extended keyboard mapping specifications used by the X Keyboard Extension. When the xkbcommon parser processes these crafted inputs, it follows execution paths that assume certain pointer values will be non-null, but due to the incorrect handling of desupported tokens, these assumptions fail catastrophically. The vulnerability maps directly to CWE-476, which describes NULL pointer dereference conditions, and demonstrates how improper handling of deprecated features can create security weaknesses. The flaw is particularly concerning because it allows local privilege escalation to system crashes, effectively enabling denial of service attacks against applications that rely on xkbcommon for keyboard input processing. This includes desktop environments, window managers, and any software that processes keyboard configuration files, making the attack surface quite broad within graphical user interface frameworks.
The operational impact of CVE-2018-15854 extends beyond simple system crashes, as it can disrupt user sessions and potentially affect the stability of entire desktop environments. Attackers can leverage this vulnerability to cause applications to terminate unexpectedly, leading to loss of unsaved work and degraded user experience. In environments where automated processes depend on keyboard input handling, such as kiosks or embedded systems, this vulnerability could be exploited to create persistent denial of service conditions. The vulnerability's classification under the ATT&CK framework would fall under T1499.004 for Network Denial of Service and potentially T1059.007 for Command and Scripting Interpreter, as attackers might use the instability to execute other malicious payloads. The fact that this affects a core system library means that the impact cascades through multiple applications and services that depend on proper keyboard handling functionality, making it a significant concern for system administrators and security professionals responsible for maintaining desktop environments.
Mitigation strategies for CVE-2018-15854 focus primarily on updating to version 0.8.1 or later of the xkbcommon library, where the null pointer dereference has been corrected through proper handling of geometry token processing. System administrators should prioritize patching affected systems, particularly those running desktop environments or applications that process keyboard configuration files. Additional protective measures include implementing proper input validation for keyboard mapping files, using sandboxing techniques for applications that handle user-supplied keymap data, and monitoring for unusual application crashes related to keyboard input processing. The vulnerability serves as a reminder of the importance of proper deprecation handling in security-critical libraries, where simply removing features without proper null checking can create exploitable conditions. Organizations should also consider implementing automated patch management systems to ensure timely updates of core system libraries, as this vulnerability demonstrates how seemingly benign library updates can introduce critical security issues if proper testing and validation are not performed during the development lifecycle.