CVE-2018-15858 in xkbcommon
Summary
by MITRE
Unchecked NULL pointer usage when handling invalid aliases in CopyKeyAliasesToKeymap in xkbcomp/keycodes.c 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.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability identified as CVE-2018-15858 represents a critical null pointer dereference flaw within the xkbcommon library's xkbcomp component, specifically in the CopyKeyAliasesToKeymap function located in xkbcomp/keycodes.c. This issue affects versions prior to 0.8.1 and demonstrates a classic software security weakness where improper input validation leads to memory access violations. The xkbcommon library serves as a fundamental component for X Keyboard Extension handling across numerous Linux distributions and desktop environments, making this vulnerability particularly concerning for system stability and security.
The technical flaw manifests when the CopyKeyAliasesToKeymap function processes invalid alias entries within keymap files without proper null pointer checks. When malformed keymap data containing invalid aliases is processed, the function attempts to dereference a null pointer that should have been validated before use. This unchecked pointer usage creates a predictable crash condition that can be exploited by local attackers who craft malicious keymap files. The vulnerability operates at the parser level where input validation fails to account for edge cases in alias handling, specifically when alias references point to null or invalid memory locations.
The operational impact of this vulnerability extends beyond simple denial of service, as local attackers can leverage this weakness to cause system instability and potentially disrupt graphical user interfaces that depend on proper keyboard mapping. Since xkbcommon is widely used across various desktop environments and windowing systems, a successful exploitation could affect multiple applications and services that rely on keyboard input handling. The null pointer dereference creates a crash condition that typically results in process termination, but in some scenarios could potentially be extended to more sophisticated attack vectors depending on the execution context and surrounding system components.
Mitigation strategies for CVE-2018-15858 primarily focus on upgrading to xkbcommon version 0.8.1 or later, which includes proper null pointer validation in the CopyKeyAliasesToKeymap function. System administrators should prioritize patching affected systems and monitoring for any unauthorized access attempts that might indicate exploitation attempts. Additionally, implementing proper input validation and sanitization measures for any user-supplied keymap files can provide defense-in-depth protection. The vulnerability aligns with CWE-476 which specifically addresses NULL pointer dereference conditions, and could potentially map to ATT&CK technique T1059 for execution through command-line interfaces where keyboard mapping might be involved in system operations. Organizations should also consider implementing access controls that limit the ability of untrusted users to provide keymap files that could trigger this vulnerability.