CVE-2018-15855 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 the XkbFile for an xkb_geometry section was mishandled.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15855 represents a critical null pointer dereference flaw within the xkbcommon library version 0.8.1 and earlier. This issue affects the X Keyboard Extension common library which is widely used across Linux systems for parsing and processing keyboard mapping files. The vulnerability stems from improper handling of XkbFile structures when processing xkb_geometry sections within keymap files, creating a scenario where a maliciously crafted input can trigger a crash condition. The xkbcommon library serves as a fundamental component in keyboard input handling across various desktop environments and windowing systems, making this vulnerability particularly concerning for system stability and security.
The technical implementation of this vulnerability occurs when the xkbcommon parser encounters a malformed keymap file containing an xkb_geometry section with improperly initialized or null XkbFile structures. During the parsing process, the library attempts to dereference a pointer that has not been properly validated or initialized, resulting in an immediate system crash. This behavior aligns with CWE-476 which specifically addresses null pointer dereference vulnerabilities, where a null pointer is used in a context that requires a valid object reference. The flaw demonstrates poor input validation and error handling practices within the library's parsing logic, particularly when dealing with malformed geometry section data that should be gracefully handled rather than causing system termination.
From an operational impact perspective, this vulnerability provides local attackers with a reliable method for causing denial of service conditions within systems that utilize xkbcommon for keyboard processing. The crash occurs during the parsing phase of keymap files, which means that any application or system component that loads keyboard configuration data could be affected. This includes desktop environments like GNOME and KDE, window managers, and system utilities that process keyboard mappings. The vulnerability's accessibility makes it particularly dangerous as it requires minimal privileges to exploit and can affect system stability even when the attacker cannot directly control the execution environment. The ATT&CK framework categorizes this as a privilege escalation technique through service or application exploitation, where the null pointer dereference results in system instability and potential service disruption.
Mitigation strategies for this vulnerability involve immediate upgrading to xkbcommon version 0.8.1 or later where the null pointer dereference has been addressed through proper input validation and initialization checks. System administrators should prioritize patching affected systems, particularly those running desktop environments or applications that heavily rely on keyboard mapping functionality. Additional defensive measures include implementing proper input sanitization for any keymap files processed by applications, monitoring for unusual crash patterns that might indicate exploitation attempts, and maintaining regular security updates for all system components that utilize xkbcommon. The fix implemented in version 0.8.1 specifically addresses the improper handling of XkbFile structures by ensuring proper validation and initialization before pointer dereference operations, thereby preventing the null pointer dereference condition that previously led to system crashes.