AppInit DLLs

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AppInit DLLs
Technique
ID T1103
Tactic Persistence, Privilege Escalation
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1
Permissions Required Administrator
Effective Permissions Administrator, SYSTEM
Data Sources Loaded DLLs, Process monitoring, Windows Registry

DLLs that are specified in the AppInit_DLLs value in the Registry key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows are loaded by user32.dll into every process that loads user32.dll. In practice this is nearly every program. This value can be abused to obtain persistence by causing a DLL to be loaded into most processes on the computer.1

The AppInit DLL functionality is disabled in Windows 8 and later versions when secure boot is enabled.2

Examples

  • If a victim meets certain criteria, T9000 uses the AppInit_DLL functionality to achieve persistence by ensuring that every user mode process that is spawned will load its malicious DLL, ResN32.dll. It does this by creating the following Registry keys: HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_DLLs – %APPDATA%\Intel\ResN32.dll and HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows\LoadAppInit_DLLs – 0x1.3
  • Some variants of Cherry Picker use AppInit_DLLs to achieve persistence by creating the following Registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows "AppInit_DLLs"="pserver32.dll"4

Mitigation

Upgrade to Windows 8 or later and enable secure boot.

Identify and block potentially malicious software that may be executed through AppInit DLLs by using whitelisting5 tools, like AppLocker,67 that are capable of auditing and/or blocking unknown DLLs.

Detection

Monitor DLL loads by processes that load user32.dll and look for DLLs that are not recognized or not normally loaded into a process. Monitor the AppInit_DLLs Registry value for modifications that do not correlate with known software, patch cycles, etc. Tools such as Sysinternals Autoruns may also be used to detect system changes that could be attempts at persistence, including listing current AppInit DLLs.8

Look for abnormal process behavior that may be due to a process loading a malicious DLL. Data and events should not be viewed in isolation, but as part of a chain of behavior that could lead to other activities, such as making network connections for Command and Control, learning details about the environment through Discovery, and conducting Lateral Movement.