Process Hollowing

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Process Hollowing
Technique
ID T1093
Tactic Defense Evasion, Execution
Platform Windows Server 2003, Windows Server 2008, Windows Server 2012, Windows XP, Windows 7, Windows 8, Windows Server 2003 R2, Windows Server 2008 R2, Windows Server 2012 R2, Windows Vista, Windows 8.1, Windows 10
Permissions Required User
Data Sources Process monitoring, API monitoring, Process monitoring
Supports Remote No
Defense Bypassed Process whitelisting, Anti-virus, Whitelisting by file name or path, Signature-based detection

Process hollowing occurs when a process is created in a suspended state and the process's memory is replaced with the code of a second program so that the second program runs instead of the original program. Windows and process monitoring tools believe the original process is running, whereas the actual program running is different.1

Process hollowing may be used similarly to DLL Injection to evade defenses and detection analysis of malicious process execution by launching adversary-controlled code under the context of a legitimate process.

Examples

  • A Patchwork payload uses process hollowing to hide the UAC bypass vulnerability exploitation inside svchost.exe.2
  • BADNEWS has a command to download an .exe and use process hollowing to inject it into a new process.3
  • BBSRAT has been seen loaded into msiexece.exe through process hollowing to hide its execution.4
  • Duqu is capable of loading executable code via process hollowing.5

Mitigation

Mitigating specific API calls will likely have unintended side effects, such as preventing legitimate software from operating properly. Efforts should be focused on preventing adversary tools from running earlier in the chain of activity and on identifying subsequent malicious behavior.

Although process hollowing may be used to evade certain types of defenses, it is still good practice to identify potentially malicious software that may be used to perform adversarial actions, including process hollowing, and audit and/or block it by using whitelisting6 tools, like AppLocker,78 or Software Restriction Policies9 where appropriate.10

Detection

Monitoring API calls may generate a significant amount of data and may not be directly useful for defense unless collected under specific circumstances for known bad sequences of calls, since benign use of API functions may be common and difficult to distinguish from malicious behavior.

Analyze process behavior to determine if a process is performing actions it usually does not, such as opening network connections, reading files, or other suspicious actions that could relate to post-compromise behavior.