CVE-2015-0856 in sddm
Summary
by MITRE
daemon/Greeter.cpp in sddm before 0.13.0 does not properly disable the KDE crash handler, which allows local users to gain privileges by crashing a greeter when using certain themes, as demonstrated by the plasma-workspace breeze theme.
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Analysis
by VulDB Data Team • 06/27/2022
The vulnerability identified as CVE-2015-0856 resides within the sddm display manager software version 0.13.0 and earlier, specifically in the daemon/Greeter.cpp component. This flaw represents a privilege escalation vulnerability that exploits the improper handling of crash events within the graphical greeter interface. The issue manifests when sddm fails to adequately disable the KDE crash handler mechanism, creating an exploitable condition that local attackers can leverage to elevate their privileges on the system. The vulnerability is particularly significant because it operates at the display manager level, which serves as the initial point of user interaction with the graphical system, making it an attractive target for attackers seeking persistent system access.
The technical root cause of this vulnerability stems from the sddm greeter component's failure to properly isolate crash handling mechanisms during the graphical login process. When certain themes such as the plasma-workspace breeze theme are used, the greeter process maintains references to KDE crash handler components that should be disabled for security reasons. This oversight allows local users to manipulate the greeter into crashing in a manner that triggers the crash handler, which operates with elevated privileges due to the display manager's role in the system boot process. The flaw specifically relates to improper privilege separation and the failure to implement proper sandboxing or privilege reduction mechanisms for the greeter process during crash scenarios. This vulnerability aligns with CWE-276, which addresses improper privilege management, and demonstrates weaknesses in secure privilege handling within graphical interfaces.
The operational impact of CVE-2015-0856 extends beyond simple privilege escalation to encompass broader system compromise capabilities. Attackers who successfully exploit this vulnerability can gain elevated privileges that allow them to execute arbitrary code with system-level access, potentially leading to complete system compromise. The exploitation requires local access to the target system, making it particularly concerning for environments where local user accounts may be compromised or where attackers can obtain physical access to the machine. The vulnerability is particularly dangerous because it operates at the login interface level, where users typically have elevated privileges due to the display manager's role in system authentication. This attack vector can be particularly effective in targeted attacks against desktop environments where sddm is the primary display manager, as it provides a path to system-level access that bypasses normal authentication mechanisms.
The exploitation of this vulnerability can be mapped to several ATT&CK techniques including privilege escalation through process injection and abuse of system privileges. The attack chain typically involves manipulating the greeter process to crash in a controlled manner, leveraging the enabled crash handler to execute code with elevated privileges. This aligns with ATT&CK tactic TA0004 (Privilege Escalation) and technique T1068 (Local Port Knocking) where the system's crash handling mechanism is abused. The vulnerability also demonstrates weaknesses in secure system design and proper privilege management, which can be addressed through proper security hardening of display managers and the implementation of proper process isolation mechanisms. Organizations should implement immediate mitigations including upgrading to sddm version 0.13.0 or later, where the vulnerability has been patched, and ensuring proper configuration of display manager crash handling mechanisms to prevent privilege escalation attacks. The fix in the patched version addresses the core issue by properly disabling the KDE crash handler within the greeter context, preventing local users from exploiting the privilege escalation path through controlled crash scenarios.
This vulnerability highlights the critical importance of proper privilege management and secure coding practices within graphical system components that interface with user sessions. The issue demonstrates how seemingly minor oversights in crash handling can create significant security implications, particularly in display managers that operate with elevated privileges. System administrators should conduct thorough security assessments of their display manager configurations and ensure that all graphical components properly isolate user processes and disable unnecessary system functionality that could be exploited by local attackers. The vulnerability also underscores the need for comprehensive security testing of graphical interfaces and authentication systems, as these components often serve as critical attack vectors for privilege escalation and system compromise attacks.