AppInit DLLs

Dynamic-link libraries (DLLs) that are specified in the AppInit_DLLs value in the Registry keys HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Windows or HKEY_LOCAL_MACHINE\Software\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows 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. [1] Similar to Process Injection, these values can be abused to obtain persistence and privilege escalation by causing a malicious DLL to be loaded and run in the context of separate processes on the computer. [2]

The AppInit DLL functionality is disabled in Windows 8 and later versions when secure boot is enabled. [3]

ID: T1103

Tactic: Persistence, Privilege Escalation

Platform:  Windows

System Requirements:  Secure boot disabled on systems running Windows 8 and later

Permissions Required:  Administrator

Effective Permissions:  Administrator, SYSTEM

Data Sources:  Loaded DLLs, Process monitoring, Windows Registry

Version: 1.0

Mitigations

Mitigation Description
Execution Prevention Adversaries can install new AppInit_DLLs binaries to execute this technique. Identify and block potentially malicious software executed through AppInit_DLLs functionality by using application whitelisting tools, like Windows Defender Application Control, AppLocker, or Software Restriction Policies where appropriate.
Update Software Upgrade to Windows 8 or later and enable secure boot.

Examples

Name Description
Cherry Picker

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]

T9000

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.[5]

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 values for modifications that do not correlate with known software, patch cycles, etc. Monitor and analyze application programming interface (API) calls that are indicative of Registry edits such as RegCreateKeyEx and RegSetValueEx. [1] Tools such as Sysinternals Autoruns may also be used to detect system changes that could be attempts at persistence, including listing current AppInit DLLs. [6]

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.

References