Winlogon Helper DLL

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

Winlogon.exe is a Windows component responsible for actions at logon/logoff as well as the secure attention sequence (SAS) triggered by Ctrl-Alt-Delete. Registry entries in HKLM\Software\[Wow6432Node\]Microsoft\Windows NT\CurrentVersion\Winlogon\ and HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\ are used to manage additional helper programs and functionalities that support Winlogon.1

Malicious modifications to these Registry keys may cause Winlogon to load and execute malicious DLLs and/or executables. Specifically, the following subkeys have been known to be possibly vulnerable to abuse:1

  • Winlogon\Notify - points to notification package DLLs that handle Winlogon events
  • Winlogon\Userinit - points to userinit.exe, the user initialization program executed when a user logs on
  • Winlogon\Shell - points to explorer.exe, the system shell executed when a user logs on

Adversaries may take advantage of these features to repeatedly execute malicious code and establish Persistence.

Examples

  • A Dipsind variant registers as a Winlogon Event Notify DLL to establish persistence.2
  • Gazer can establish persistence by setting the value “Shell” with “explorer.exe, %malware_pathfile%” under the Registry key HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon.3

Mitigation

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 whitelisting4 tools like AppLocker56 that are capable of auditing and/or blocking unknown DLLs.

Detection

Monitor for changes to Registry entries associated with Winlogon 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.7 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.