As we know, malware becomes stealthier by somehow achieving persistence on the exploited machine. This helps malware authors to inject/exploit once, and the malware will continue to act even after restarts/reboots/log-offs, etc. In this article, we will focus only on Windows as it has a lot of areas like Autostart Extension Points (ASEP) through which the persistence can be achieved. This article will contain the common ways of achieving it and is by no means is an exhaustive list to achieve persistence on a Windows machine.
Below are the common ways of achieving Persistence used by Malware
- Modifying Registry Keys
Modifying registry keys are often used by malware to achieve persistence on a system. Below are some of the most common registry values/locations exploited by malware.
As I stated above windows has a lot of AutoStart Extension Points(ASEP). When it comes to malware, most of them would like to achieve persistence by editing the below registry keys:
Above listed keys are at the user level and are often used by malware to achieve persistence if not able to exploit the admin/system level privileges.
Otherwise, malware infects these keys at system level
Since smss.exe launches before windows subsystem loads, it calls configuration subsystem to load the hive present at HKLM\SYSTEM\CurrentControlSet\Control\hivelist. Also smss.exe will launch anything present in the BootExecute key at HKEY_LOCAL_MACHINE\SYSTEM\ControlSet002\Control\Session Manager. It should always have the value of autocheck autochk*. If there are more values in it, then probably the malware is likely to launch at boot.
Keys used by WinLogon Process
- Userinit Key
Winlogon process uses the value specified in the Userinit key to launch login scripts etc. This key is location at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon. Usually, userinit key points to userinit.exe but if this key can be altered, then that exe will also launch by Winlogon.
Since Winlogon handles Secure Attention Sequence (SAS) (Ctrl+Alt+Del), notify subkeys found at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify are used to notify event handles when SAS happens and loads a DLL. This DLL can be edited to launch whenever such SAS event occurs.
Pointed by key located at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell, this key points to explorer.exe(Windows interface) and should only be string explorer.exe rather than complete path as it is supposed to launch from \windows. The boot key at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\IniFileMapping\system.ini\boot points to the location under Winlogon only.
Placing a malicious file under the startup directory is often used by malware authors. Any shortcut created to the location pointed by subkey Startup will launch the service during logon/reboot. Start up location is specified both at Local Machine and Current User.
- HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
- HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
- HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
- HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
- Many windows services are required to run at boot like Workstation/server services, Windows Event Log, and other Win drivers. These are located at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services.
Along with placing a malicious file in the above-listed registry key, there is another way to load malicious files. Malicious files can be loaded if a service fails to start. For example below screenshot shows how RDP failure can be used to run a program.
There are some other keys which are used to start background services like remote registry service. These are located at:
Browser Helper Objects(BHO)
- It is essentially a DLL module loaded when Internet Explorer starts up. Various data theft types malware affect BHO. They are located at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects. There are various subkeys under BHO which tell the browser to load which DLLs.
- Key located at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs will show the DLLs loaded by the User32.dll. As most executables load User32.dll, this is a good place for malicious DLLs to reside.
File Association keys
- Located at HKEY_LOCAL_MACHINE\Software\Classes\ and HKEY_CLASSES_ROOT\; there are various keys which are used to specify the action when a certain type of files are open. For example below is the Command value when a txt file is opened in my system HKEY_CLASSES_ROOT\textfile\shell\open\command
DLL Search Order Hijacking
Another common method used by malware is to hijack a concept about how the OS loads DLLs. Whenever an exe loads (even explorer.exe), it follows a certain path search to load the required DLLs. Because DLLs are loaded in the order the directories are parsed, it is possible to add a malicious DLL with the same name in a directory earlier than the directory where the legit DLL resides. If Safe DLL search mode is enabled (which is by default on most versions) then OS will check whether the DLL is already loaded in memory or is it a part of Known DLLs registry key located at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\KnownDLLs. If OS cannot find the DLL at either of these, then DLL search starts in the following order
- Directory from where application was launched
- System Directory(C:\Windows\System32)
- Windows Directory
- Current Working Directory
- Directories defined in the PATH variable.
So a malware can easily place a malicious DLL in the search order. More details about this can be found here https://msdn.microsoft.com/en-us/library/windows/desktop/ms682586(v=vs.85).aspx
Another simple but very effective technique is to hijack the shortcut icons Target attribute. Along with a normal application to be launched, shortcut icon can be forced to download content from an evil site.
Note that there are various other methods like infecting MBR, COM object hijack, etc. are also by malware, but above are some of the common method used by malware to achieve persistence.