Adversaries may establish persistence and/or elevate privileges by executing malicious content triggered by AppInit DLLs loaded into processes. Dynamic-link libraries (DLLs) that are specified in the <code>AppInit_DLLs</code> value in the Registry keys <code>HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows</code> or <code>HKEY_LOCAL_MACHINE\Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows</code> are loaded by user32.dll into every process that loads user32.dll. In practice this is nearly every program, since user32.dll is a very common library. (Citation: Elastic Process Injection July 2017)

Similar to Process Injection, these values can be abused to obtain elevated privileges by causing a malicious DLL to be loaded and run in the context of separate processes on the computer. (Citation: AppInit Registry) Malicious AppInit DLLs may also provide persistence by continuously being triggered by API activity. 

The AppInit DLL functionality is disabled in Windows 8 and later versions when secure boot is enabled. (Citation: AppInit Secure Boot)
AppInit DLLs
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1546/010  external_id: T1546.010     source_name: Elastic Process Injection July 2017  description: Hosseini, A. (2017, July 18). Ten Process Injection Techniques: A Technical Survey Of Common And Trending Process Injection Techniques. Retrieved December 7, 2017.  url: https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-process     source_name: AppInit Registry  description: Microsoft. (2006, October). Working with the AppInit_DLLs registry value. Retrieved July 15, 2015.  url: https://support.microsoft.com/en-us/kb/197571     source_name: AppInit Secure Boot  description: Microsoft. (n.d.). AppInit DLLs and Secure Boot. Retrieved July 15, 2015.  url: https://msdn.microsoft.com/en-us/library/dn280412     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     