Detect Winlogon Helper DLL in Google Chronicle
Adversaries may abuse features of Winlogon to execute DLLs and/or executables when a user logs in. 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\Microsoft\Windows NT\CurrentVersion\Winlogon\ and HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\ are used to manage additional helper programs. Malicious modifications to these Registry keys may cause Winlogon to load and execute malicious DLLs and/or executables. Specifically, the Winlogon\Notify, Winlogon\Userinit, and Winlogon\Shell subkeys are known targets for abuse by threat actors including Turla, Wizard Spider, and LockBit.
MITRE ATT&CK
- Tactic
- Persistence Privilege Escalation
- Technique
- T1547 Boot or Logon Autostart Execution
- Sub-technique
- T1547.004 Winlogon Helper DLL
- Canonical reference
- https://attack.mitre.org/techniques/T1547/004/
YARA-L Detection Query
rule winlogon_helper_dll_persistence {
meta:
author = "Argus Detection Engineering"
description = "Detects modifications to Winlogon registry keys (Shell, Userinit, Notify, VmApplet, AppSetup, Taskman) that may indicate Winlogon Helper DLL persistence (T1547.004). Adversary groups including Turla, Wizard Spider, and LockBit abuse these keys."
mitre_attack_tactic = "Persistence"
mitre_attack_technique = "T1547.004"
severity = "HIGH"
confidence = "HIGH"
reference = "https://attack.mitre.org/techniques/T1547/004/"
events:
$e.metadata.event_type = "REGISTRY_MODIFICATION"
$e.target.registry.registry_key = /(?i)\\Windows NT\\CurrentVersion\\Winlogon/
$e.target.registry.registry_value_name = /(?i)^(Shell|Userinit|Notify|VmApplet|AppSetup|Taskman)$/
not (
$e.target.registry.registry_value_name = "Shell" and
$e.target.registry.registry_value_data = "explorer.exe"
)
not (
$e.target.registry.registry_value_name = "Userinit" and
$e.target.registry.registry_value_data = /(?i)C:\\Windows\\system32\\userinit\.exe,/
)
condition:
$e
}Chronicle YARA-L 2.0 rule detecting modifications to known Winlogon persistence subkeys. Uses UDM registry event fields to match key path, value name, and filters out legitimate default data. Covers both HKLM and HKCU hive paths. Fires on any non-default write to Shell, Userinit, Notify, VmApplet, AppSetup, or Taskman under the Winlogon key.
Data Sources
Required Tables
False Positives & Tuning
- Vendor software performing in-place OS upgrades that temporarily set a custom Shell value during the upgrade phase
- Enterprise single sign-on (SSO) tools that register a Winlogon Notify DLL for credential interception on corporate workstations
- Kiosk mode deployments that intentionally set a custom Shell value (e.g., a restricted browser launcher) as an approved configuration
Other platforms for T1547.004
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Modify Winlogon Shell to Append Malicious Binary
Expected signal: Sysmon Event ID 13: RegistryValueSet on HKCU\...\Winlogon\Shell with value containing 'explorer.exe, C:\Windows\System32\calc.exe'.
- Test 2Modify Winlogon Userinit to Chain Malicious Binary
Expected signal: Sysmon Event ID 13: RegistryValueSet on HKLM\...\Winlogon\Userinit showing the appended executable path.
- Test 3Create Winlogon Notify Key
Expected signal: Sysmon Event ID 12: Key created for the Notify subkey. Sysmon Event ID 13: Value set for DllName.
References (4)
- https://attack.mitre.org/techniques/T1547/004/
- https://web.archive.org/web/20160214140250/http://blog.cylance.com/windows-registry-persistence-part-2-the-run-keys-and-search-order
- https://technet.microsoft.com/en-us/sysinternals/bb963902
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1547.004/T1547.004.md
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