CVE-2013-4394 in systemd
Summary
by MITRE
The SetX11Keyboard function in systemd, when PolicyKit Local Authority (PKLA) is used to change the group permissions on the X Keyboard Extension (XKB) layouts description, allows local users in the group to modify the Xorg X11 Server configuration file and possibly gain privileges via vectors involving "special and control characters."
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Analysis
by VulDB Data Team • 01/09/2022
The vulnerability identified as CVE-2013-4394 resides within the systemd component responsible for managing X11 keyboard configurations through the SetX11Keyboard function. This flaw specifically manifests when PolicyKit Local Authority (PKLA) mechanisms are employed to adjust group permissions for X Keyboard Extension (XKB) layout descriptions. The security implications arise from the improper handling of special and control characters within the Xorg X11 Server configuration file modification process, creating a privilege escalation vector that can be exploited by local users who belong to the affected group.
The technical implementation of this vulnerability stems from insufficient input validation and sanitization within the SetX11Keyboard function. When users with appropriate group membership attempt to modify XKB layout configurations, the system fails to properly sanitize special and control characters that may be embedded within the configuration data. This inadequate sanitization allows attackers to inject malicious sequences that can manipulate the Xorg server configuration file, potentially leading to unauthorized privilege escalation. The vulnerability operates under CWE-20, which describes improper input validation, and more specifically aligns with CWE-78, representing command injection vulnerabilities that can occur when user-supplied data is not properly escaped or filtered.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it provides attackers with the ability to modify critical system configuration files that govern X11 keyboard behavior. This modification capability can be leveraged to establish persistent access mechanisms or to disable security features within the X11 subsystem. Attackers can exploit the vulnerability by crafting specially formatted XKB layout descriptions containing control characters that manipulate the configuration file parsing process, potentially leading to arbitrary code execution within the context of the Xorg server or related processes. The attack surface is particularly concerning because it operates at the system level where X11 configurations are processed, allowing for both local privilege escalation and potential denial of service conditions.
From a threat modeling perspective, this vulnerability maps to several ATT&CK techniques including T1068, which involves exploiting legitimate credentials and privileges to gain access to systems, and T1548.001, which covers abuse of group privileges for privilege escalation. The vulnerability is particularly dangerous in multi-user environments where local users have access to group memberships that grant them rights to modify X11 keyboard configurations. Mitigation strategies should include implementing proper input sanitization and validation within the SetX11Keyboard function, restricting group membership for users who do not require X11 keyboard configuration privileges, and applying proper access controls to Xorg configuration files. Additionally, system administrators should consider disabling unnecessary PKLA rules that grant excessive permissions for X11 keyboard management and ensure that all system updates are applied promptly to remediate this vulnerability. The vulnerability demonstrates the critical importance of proper input validation in system-level components that interact with configuration files and privileged operations.