T1004 - Winlogon Helper DLL

Winlogon.exe is a Windows component responsible for actions at logon/logoff as well as the secure attention sequence (SAS) triggered by Ctrl-Alt-Delete. Registry entries in HKLM\Software[Wow6432Node]Microsoft\Windows NT\CurrentVersion\Winlogon</code> and HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon</code> are used to manage additional helper programs and functionalities that support Winlogon. (Citation: Cylance Reg Persistence Sept 2013)

Malicious modifications to these Registry keys may cause Winlogon to load and execute malicious DLLs and/or executables. Specifically, the following subkeys have been known to be possibly vulnerable to abuse: (Citation: Cylance Reg Persistence Sept 2013)

• Winlogon\Notify - points to notification package DLLs that handle Winlogon events
• Winlogon\Userinit - points to userinit.exe, the user initialization program executed when a user logs on
• Winlogon\Shell - points to explorer.exe, the system shell executed when a user logs on

Adversaries may take advantage of these features to repeatedly execute malicious code and establish Persistence.