After adversaries have implanted a webshell, it’s easy for them to gain a reverse shell via tools such as netcat, the Metasploit framework, or Powercat. The W3WP process itself is the IIS process running within Exchange. Whenever that process spawns the cmd.exe or powershell.exe processes, you have a match. The reverse shell allows the target machine to communicate back to the attacking machine. The attacking machine has a listener port on which it receives the connection, which by using, code or command execution is achieved. You would like an automated way to terminate W3WP spawned processes.
How to use Splunk software for this use case
Playbook: Delete detected files terminates the process spawned by the W3WP process. Then, the format block creates SPL to search Splunk SOAR for any child processes of the process identified in the detection. Splunk SOAR runs the Splunk search to find the process IDs of child processes that were run. Finally, the playbook runs the terminate process command for any child processes that were found in the previous search. To use the playbook:
- Configure and enable Splunk Enterprise Security Content Updates and the Splunk Add-on for Phantom.
- Run the W3WP Spawning Shell detection in the HAFNIUM Group analytic story in Splunk Enterprise Security.
- Enable the Send to Phantom Adaptive Response Action in the Enterprise Security correlation search. After a web shell is written, the detection sends the event to Splunk SOAR.
- If you haven't previously used this playbook, configure and activate it.
- Navigate to Home > Playbooks and search for terminate spawned processes. If it’s not there, click Update from Source Control and select Community to download new community playbooks.
- Click the playbook name to open it.
- Resolve the playbook import wizard and set the playbook to Active.
- Save the playbook and then run it.
- Adjust the playbook as needed, according to your results.
The process that you want to terminate could actually be a child process of the one picked up in the detection. For example, the adversary could launch Powershell with a command like “cmd /c powershell.exe”. The detection would pick up the execution of cmd, so you'd want the playbook to look for both that process and any child processes that have been run.
The PowerShell command is reversed back to the listening instance, and is then terminated due to the playbook being run to terminate it. Adversaries will attempt to perform additional actions like create persistence, move laterally, and more, but automated processes in your kit bag can help deal with issues very quickly when they do pop up.
These additional Splunk resources might help you understand and implement this use case: