# Winlogon Helper DLL

Winlogon Helper DLL
Technique
ID T1004
Tactic Persistence
Platform Windows Server 2003, Windows XP, Windows Server 2003 R2
Permissions Required Administrator, SYSTEM
Data Sources Windows Registry, File monitoring, Process monitoring
CAPEC ID CAPEC-579

Winlogon is a part of some Windows versions that performs actions at logon. In Windows systems prior to Windows Vista, a Registry key can be modified that causes Winlogon to load a DLL on startup. Adversaries may take advantage of this feature to load adversarial code at startup for persistence.

## Mitigation

Upgrade the operating system to a newer version of Windows if using a version prior to Vista.

Limit the privileges of user accounts so that only authorized administrators can perform Winlogon helper changes.

Identify and block potentially malicious software that may be executed through the Winlogon helper process by using whitelisting1 tools like AppLocker23 that are capable of auditing and/or blocking unknown DLLs.

## Detection

Monitor for changes to registry entries in HKLM\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Notify 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 Winlogon helper values.4 New DLLs written to System32 that do not correlate with known good software or patching may also be suspicious.

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 network connections made for Command and Control, learning details about the environment through Discovery, and Lateral Movement.