AppCert DLLs

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AppCert DLLs
Technique
ID T1182
Tactic Persistence, Privilege Escalation
Platform Windows
Permissions Required Administrator, SYSTEM
Effective Permissions Administrator, SYSTEM
Data Sources Loaded DLLs, Process Monitoring, Windows Registry

Dynamic-link libraries (DLLs) that are specified in the AppCertDLLs value in the Registry key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager are loaded into every process that calls the ubiquitously used application programming interface (API) functions:1

  • CreateProcess
  • CreateProcessAsUser
  • CreateProcessWithLoginW
  • CreateProcessWithTokenW
  • WinExec

Similar to Process Injection, this value can be abused to obtain persistence and privilege escalation by causing a malicious DLL to be loaded and run in the context of separate processes on the computer.

Mitigation

Identify and block potentially malicious software that may be executed through AppCert DLLs by using whitelisting2 tools, like AppLocker,34 that are capable of auditing and/or blocking unknown DLLs.

Detection

Monitor DLL loads by processes, specifically looking for DLLs that are not recognized or not normally loaded into a process. Monitor the AppCertDLLs Registry value for modifications that do not correlate with known software, patch cycles, etc. Monitor and analyze application programming interface (API) calls that are indicative of Registry edits such as RegCreateKeyEx and RegSetValueEx.1

Tools such as Sysinternals Autoruns may overlook AppCert DLLs as an auto-starting location.56

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