File System Permissions Weakness

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File System Permissions Weakness
Technique
ID T1044
Tactic Persistence, Privilege Escalation
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1
Permissions Required User, Administrator
Effective Permissions User, Administrator, SYSTEM
Data Sources File monitoring, Process command-line parameters, Services
Contributors Stefan Kanthak

Processes may automatically execute specific binaries as part of their functionality or to perform other actions. If the permissions on the file system directory containing a target binary, or permissions on the binary itself, are improperly set, then the target binary may be overwritten with another binary using user-level permissions and executed by the original process. If the original process and thread are running under a higher permissions level, then the replaced binary will also execute under higher-level permissions, which could include SYSTEM.

Adversaries may use this technique to replace legitimate binaries with malicious ones as a means of executing code at a higher permissions level. If the executing process is set to run at a specific time or during a certain event (e.g., system bootup) then this technique can also be used for persistence.

Services

Manipulation of Windows service binaries is one variation of this technique. Adversaries may replace a legitimate service executable with their own executable to gain persistence and/or privilege escalation to SYSTEM. Once the service is started, either directly by the user (requiring administrator privileges) or through some other means, such as a system restart if the service starts on bootup, the replaced executable will run instead of the original service executable.

Examples

  • One variant of BlackEnergy locates existing driver services that have been disabled and drops its driver component into one of those service's paths, replacing the legitimate executable. The malware then sets the hijacked service to start automatically in order to establish persistence.1

Mitigation

Use auditing tools capable of detecting file system permissions abuse opportunities on systems within an enterprise and correct them. Limit privileges of user accounts and groups so that only authorized administrators can interact with service changes and service binary target path locations. Toolkits like the PowerSploit framework contain PowerUp modules that can be used to explore systems for service file system permissions weaknesses.2

Identify and block potentially malicious software that may be executed through service abuse by using whitelisting3 tools, like AppLocker,45 that are capable of auditing and/or blocking unknown programs.

Detection

Look for changes to binaries and service executables that may normally occur during software updates. If an executable is written, renamed, and/or moved to match an existing service executable, it could be detected and correlated with other suspicious behavior. Hashing of binaries and service executables could be used to detect replacement against historical data.

Look for abnormal process call trees from typical processes and services and for execution of other commands that could relate to Discovery or other adversary techniques.