CVE-2018-15864 in xkbcommon
Summary
by MITRE
Unchecked NULL pointer usage in resolve_keysym in xkbcomp/parser.y 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, because a map access attempt can occur for a map that was never created.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15864 represents a critical NULL pointer dereference flaw within the xkbcommon library's xkbcomp parser component. This issue affects versions prior to 082 and stems from improper handling of keymap file processing where the resolve_keysym function in parser.y fails to validate map creation status before attempting access operations. The flaw exists in the X Keyboard Extension common library which serves as a foundational component for keyboard handling across various linux desktop environments and applications. The vulnerability specifically manifests when processing crafted keymap files that contain malformed keyboard mapping definitions, creating a scenario where the parser attempts to access memory locations that were never properly initialized or allocated. This represents a classic example of improper null pointer dereference as classified under CWE-476, where the software references a pointer that has a null value without proper validation. The attack vector is particularly concerning for local adversaries who can craft malicious keymap files to exploit this condition and cause denial of service through application crashes.
The technical implementation of this vulnerability occurs within the xkbcomp parser's key symbol resolution mechanism where the resolve_keysym function performs map access operations without verifying that the target map structure has been successfully created and populated. When a malformed keymap file is processed, the parser logic flow can lead to situations where a map variable remains NULL while subsequent code attempts to access its members or perform operations on it. This particular flaw demonstrates poor defensive programming practices and highlights the importance of proper input validation and state checking in parsing components. The vulnerability is particularly dangerous because it affects the core keyboard mapping infrastructure that many applications depend upon, making it a prime target for privilege escalation or persistent denial of service attacks. The flaw operates at the level of the X Keyboard Extension parser and affects any application or system component that utilizes xkbcommon for keyboard configuration processing, including desktop environments like gnome, kde, and xfce, as well as various terminal emulators and windowing systems.
The operational impact of CVE-2018-15864 extends beyond simple application crashes to potentially compromise entire desktop environments and system usability. Local attackers can exploit this vulnerability to cause persistent service disruptions, forcing users to reboot systems or manually reset keyboard configurations. In multi-user environments, this vulnerability could be leveraged to create persistent denial of service conditions that affect multiple users simultaneously. The vulnerability also poses risks to automated systems and server environments where keyboard configuration files might be processed as part of system initialization or configuration management processes. From an attacker perspective, this represents a reliable method for causing system instability without requiring elevated privileges, making it particularly attractive for disruptive attacks. The vulnerability's classification under ATT&CK technique T1499.004 for network denial of service demonstrates its potential for broader impact beyond simple local privilege escalation. Security analysts should consider this vulnerability when assessing desktop security postures and system hardening configurations, particularly in environments where users might be able to introduce or modify keyboard mapping files.
Mitigation strategies for CVE-2018-15864 primarily focus on immediate patch application to xkbcommon library versions 0.8.2 or later where the NULL pointer dereference has been addressed. System administrators should prioritize updating all affected components and verify that keyboard mapping processing is performed through patched libraries. Additional defensive measures include implementing strict input validation for keyboard configuration files, particularly those processed through automated systems or user-provided inputs. Organizations should consider implementing monitoring for unusual keyboard mapping file processing activities and establish baseline behaviors for normal system keyboard operations. The vulnerability highlights the importance of proper code review processes for parsing components and emphasizes the need for comprehensive testing of edge cases in input handling. Security teams should also consider implementing application whitelisting for keyboard configuration tools and establish procedures for validating keyboard mapping files before system deployment. Regular vulnerability scanning and patch management processes should include verification of xkbcommon library versions across all system components to prevent exploitation of this and similar parsing vulnerabilities. The fix implemented in version 0.8.2 demonstrates proper null pointer validation and map creation status checking that should serve as a model for similar parsing components throughout the software ecosystem.