Winlogon Helper DLL
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. 
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: 
- 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.
Identify and block potentially malicious software that may be executed through the Winlogon helper process by using whitelisting tools like AppLocker that are capable of auditing and/or blocking unknown DLLs.
|User Account Management||
Limit the privileges of user accounts so that only authorized administrators can perform Winlogon helper changes.
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.  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.
- Langendorf, S. (2013, September 24). Windows Registry Persistence, Part 2: The Run Keys and Search-Order. Retrieved April 11, 2018.
- Beechey, J. (2010, December). Application Whitelisting: Panacea or Propaganda?. Retrieved November 18, 2014.
- Tomonaga, S. (2016, January 26). Windows Commands Abused by Attackers. Retrieved February 2, 2016.
- NSA Information Assurance Directorate. (2014, August). Application Whitelisting Using Microsoft AppLocker. Retrieved March 31, 2016.
- Windows Defender Advanced Threat Hunting Team. (2016, April 29). PLATINUM: Targeted attacks in South and Southeast Asia. Retrieved February 15, 2018.
- ESET. (2017, August). Gazing at Gazer: Turla’s new second stage backdoor. Retrieved September 14, 2017.
- Falcone, R., Lee, B. (2018, November 20). Sofacy Continues Global Attacks and Wheels Out New ‘Cannon’ Trojan. Retrieved November 26, 2018.
- Legezo, D. (2019, January 30). Chafer used Remexi malware to spy on Iran-based foreign diplomatic entities. Retrieved April 17, 2019.
- Parys, B. (2017, February 11). The KeyBoys are back in town. Retrieved June 13, 2019.
- Hulcoop, A., et al. (2016, November 17). It’s Parliamentary KeyBoy and the targeting of the Tibetan Community. Retrieved June 13, 2019.
- ESET, et al. (2018, January). Diplomats in Eastern Europe bitten by a Turla mosquito. Retrieved July 3, 2018.
- Ray, V. (2016, November 22). Tropic Trooper Targets Taiwanese Government and Fossil Fuel Provider With Poison Ivy. Retrieved November 9, 2018.
- Russinovich, M. (2016, January 4). Autoruns for Windows v13.51. Retrieved June 6, 2016.