CVE-2018-15857 in xkbcommon
Summary
by MITRE
An invalid free in ExprAppendMultiKeysymList in xkbcomp/ast-build.c in xkbcommon before 0.8.1 could be used by local attackers to crash xkbcommon keymap parsers or possibly have unspecified other impact by supplying a crafted keymap file.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15857 represents a critical memory management flaw within the xkbcommon library's xkbcomp component, specifically in the ExprAppendMultiKeysymList function located in xkbcomp/ast-build.c. This issue affects versions prior to 0.8.1 and demonstrates a classic invalid free operation that can be exploited through carefully crafted keymap files. The xkbcommon library serves as a crucial component for handling keyboard mappings in X Window System environments, making this vulnerability particularly concerning for desktop and server systems that rely on proper keyboard input handling. The flaw occurs during the parsing of keymap files where the system attempts to free memory that has either already been freed or was never allocated through the standard allocation mechanisms, creating a potential for memory corruption.
The technical execution of this vulnerability involves local attackers who can craft malicious keymap files that trigger the invalid free operation when xkbcomp processes these inputs. This particular flaw falls under the category of memory safety issues and can be classified as a CWE-415: Double Free or CWE-416: Use After Free, depending on the specific execution path and memory state when the invalid free occurs. The vulnerability's impact extends beyond simple crashes, as the memory corruption could potentially lead to arbitrary code execution or other unspecified behaviors that could be leveraged for privilege escalation or system compromise. The xkbcomp utility is commonly used in various X11 applications and system configurations, meaning that exploitation could affect multiple attack vectors including desktop environments, X server configurations, and any application that depends on xkbcommon for keyboard mapping operations.
The operational impact of CVE-2018-15857 is significant for system administrators and security professionals managing X Window System environments. Local attackers with access to the system can exploit this vulnerability to cause denial of service through crashes, potentially leading to system instability or complete service interruption. In more sophisticated attack scenarios, the memory corruption could enable attackers to execute arbitrary code with the privileges of the affected process, which typically runs with elevated permissions during X server operations or keymap processing. The vulnerability's presence in the core xkbcommon library means that any application or service that parses keymap files or interacts with keyboard mapping functionality could be affected, including desktop environments like GNOME, KDE, and XFCE, as well as server applications that handle keyboard input or configuration. The ATT&CK framework categorizes this type of vulnerability under privilege escalation and denial of service tactics, with potential for lateral movement if the affected processes have broader system access.
Mitigation strategies for CVE-2018-15857 primarily focus on upgrading to xkbcommon version 0.8.1 or later, which contains the necessary patches to address the invalid free operation. System administrators should prioritize patching all affected systems, particularly those running X Window System environments where keyboard input handling is critical. Additional protective measures include implementing proper input validation for any keymap files processed by applications, utilizing sandboxing techniques for keymap processing, and monitoring for unusual system behavior that might indicate exploitation attempts. Organizations should also consider implementing automated patch management systems to ensure timely deployment of security updates. The vulnerability highlights the importance of memory safety in system libraries and underscores the need for thorough code review processes, particularly for components that handle user-provided input data. Security monitoring should include detection of malformed keymap files and unusual memory allocation patterns that might indicate exploitation attempts, as these could serve as early warning signs of potential attacks targeting this class of vulnerability.