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Splunk Lantern

Spoolsv.exe writing a DLL

Some attacks such as PrintNightmare use spoolsv.exe to write a DLL (Dynamic-Link Library). This is not normal behavior for spoolsv.exe. This search detects the loaded module made by spoolsv.exe after the exploitation by checking for the suspicious DLL written to disk within a path of \spool\drivers\x64.

Procedure

Run the following search. You can optimize it by specifying an index and adjusting the time range.

sourcetype=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational OR source=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational EventID=11 
process_name=spoolsv.exe file_path="*\\spool\\drivers\\x64\\*" file_name=*.dll 
| stats count min(_time) AS firstTime max(_time) AS lastTime BY dest, UserID, process_name, file_path, file_name, TargetFilename

Search explanation

The table provides an explanation of what each part of this search achieves. You can adjust this query based on the specifics of your environment.

Splunk Search Explanation
sourcetype=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational OR 
source=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational
Search Sysmon operational data.
EventID=11 Searches for when a file has been overwritten.
process_name=spoolsv.exe file_path="*\\spool\\drivers\\x64\\*"
 file_name=*.dll 
Searches for spoolsv.exe writing a .dll within a path of \spool\drivers\x64.
| stats count min(_time) AS firstTime max(_time) AS lastTime BY dest, UserID, process_name, file_path, file_name, TargetFilename Return the first and last times this process occurred and rename those fields as shown. Then, sort first by destination and then by the rest of the fields shown.

Next steps

Ensure you filter for false positives on this search. 

During triage, isolate the endpoint and review for source of exploitation. Capture any additional file modification events.

If your results indicate an attack has occurred, the host or computer where the vulnerability is detected needs to be further investigated and remediated according to your response plan. This involves a final step of re-imaging the system with a known good system build after investigation.

Finally, you might be interested in other processes associated with the Detecting print spooler attacks use case.