Adversaries may abuse features of Winlogon to execute DLLs and/or executables when a user logs in. 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 <code>HKLM\Software[\\Wow6432Node\\]\Microsoft\Windows NT\CurrentVersion\Winlogon\</code> and <code>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.
Winlogon Helper DLL
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1547/004  external_id: T1547.004     source_name: Cylance Reg Persistence Sept 2013  description: Langendorf, S. (2013, September 24). Windows Registry Persistence, Part 2: The Run Keys and Search-Order. Retrieved April 11, 2018.  url: https://blog.cylance.com/windows-registry-persistence-part-2-the-run-keys-and-search-order     source_name: TechNet Autoruns  description: Russinovich, M. (2016, January 4). Autoruns for Windows v13.51. Retrieved June 6, 2016.  url: https://technet.microsoft.com/en-us/sysinternals/bb963902     