New DLL Added to AppInit_DLLs Registry Key

Hunt Hypothesis

DLLs that are specified in the AppInit_DLLs value in the Registry key HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows are loaded by user32.dll into every process that loads user32.dll

Detection Rule

Sigma (Original)

title: New DLL Added to AppInit_DLLs Registry Key
id: 4f84b697-c9ed-4420-8ab5-e09af5b2345d
status: test
description: DLLs that are specified in the AppInit_DLLs value in the Registry key HKLM\Software\Microsoft\Windows NT\CurrentVersion\Windows are loaded by user32.dll into every process that loads user32.dll
references:
    - https://eqllib.readthedocs.io/en/latest/analytics/822dc4c5-b355-4df8-bd37-29c458997b8f.html
author: Ilyas Ochkov, oscd.community, Tim Shelton
date: 2019-10-25
modified: 2022-12-25
tags:
    - attack.privilege-escalation
    - attack.persistence
    - attack.t1546.010
logsource:
    category: registry_event
    product: windows
detection:
    selection:
        - TargetObject|endswith:
              - '\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_Dlls'
              - '\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_Dlls'
        # Key Rename
        - NewName|endswith:
              - '\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_Dlls'
              - '\SOFTWARE\Wow6432Node\Microsoft\Windows NT\CurrentVersion\Windows\AppInit_Dlls'
    filter:
        Details: '(Empty)'
    condition: selection and not filter
falsepositives:
    - Unknown
level: medium

KQL (Azure Sentinel)

imRegistry
| where ((RegistryKey endswith "\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls" or RegistryKey endswith "\\SOFTWARE\\Wow6432Node\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls") or (RegistryKey endswith "\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls" or RegistryKey endswith "\\SOFTWARE\\Wow6432Node\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls")) and (not(RegistryValueData =~ "(Empty)"))

KQL (Microsoft 365 Defender)

DeviceRegistryEvents
| where ((RegistryKey endswith "\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls" or RegistryKey endswith "\\SOFTWARE\\Wow6432Node\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls") or (RegistryKey endswith "\\SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls" or RegistryKey endswith "\\SOFTWARE\\Wow6432Node\\Microsoft\\Windows NT\\CurrentVersion\\Windows\\AppInit_Dlls")) and (not(RegistryValueData =~ "(Empty)"))

Required Data Sources

False Positive Guidance

• Unknown

MITRE ATT&CK Context

• Tactic: Privilege Escalation
• Tactic: Persistence
• Technique: T1546.010 — T1546.010

References

• https://eqllib.readthedocs.io/en/latest/analytics/822dc4c5-b355-4df8-bd37-29c458997b8f.html
• SigmaHQ Rule