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Description

The following analytic identifies a .dll being written by spoolsv.exe. This was identified during our testing of CVE-2021-34527 previously(CVE-2021-1675) or PrintNightmare. Typically, this is not normal behavior for spoolsv.exe to write a .dll. Current POC code used will write the suspicious DLL to disk within a path of \spool\drivers\x64\. During triage, isolate the endpoint and review for source of exploitation. Capture any additional file modification events.

  • Type: TTP
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
  • Datamodel: Endpoint
  • Last Updated: 2021-07-01
  • Author: Mauricio Velazco, Michael Haag, Splunk
  • ID: 347fd388-da87-11eb-836d-acde48001122

ATT&CK

ID Technique Tactic
T1547.012 Print Processors Persistence, Privilege Escalation
`sysmon` 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, process_id 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)` 
| `spoolsv_writing_a_dll___sysmon_filter`

Associated Analytic Story

How To Implement

To successfully implement this search, you need to be ingesting logs with the process name, parent process, and command-line executions from your endpoints. If you are using Sysmon, you must have at least version 6.0.4 of the Sysmon TA. Tune and filter known instances where renamed rundll32.exe may be used.

Required field

  • _time
  • dest
  • UserID
  • process_name
  • file_path
  • file_name
  • TargetFilename

Kill Chain Phase

  • Exploitation

Known False Positives

Limited false positives. Filter as needed.

RBA

Risk Score Impact Confidence Message
72.0 80 90 $process_name$ has been identified writing dll's to $file_path$ on endpoint $dest$. This behavior is suspicious and related to PrintNightmare.

Reference

Test Dataset

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

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