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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: d5bf5cf2-da71-11eb-92c2-acde48001122

ATT&CK

ID Technique Tactic
T1547.012 Print Processors Persistence, Privilege Escalation
T1547 Boot or Logon Autostart Execution Persistence, Privilege Escalation

| tstats `security_content_summariesonly` count FROM datamodel=Endpoint.Processes where Processes.process_name=spoolsv.exe by _time Processes.process_id Processes.process_name Processes.dest 
| `drop_dm_object_name(Processes)` 
| join process_guid, _time [
| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time) as lastTime FROM datamodel=Endpoint.Filesystem where Filesystem.file_path="*\\spool\\drivers\\x64\\*" Filesystem.file_name="*.dll" by _time Filesystem.dest Filesystem.file_create_time Filesystem.file_name Filesystem.file_path 
| `drop_dm_object_name(Filesystem)` 
| fields _time dest file_create_time file_name file_path process_name process_path process] 
| dedup file_create_time 
| table dest file_create_time, file_name, file_path, process_name 
| `spoolsv_writing_a_dll_filter`

Associated Analytic Story

How To Implement

To successfully implement this search you need to be ingesting information on process that include the name of the process responsible for the changes from your endpoints into the Endpoint datamodel in the Processes node and Filesystem node.

Required field

  • _time
  • Filesystem.dest
  • Filesystem.file_create_time
  • Filesystem.file_name
  • Filesystem.file_path
  • Processes.process_name
  • Processes.process_id
  • Processes.process_name
  • Processes.dest

Kill Chain Phase

  • Exploitation

Known False Positives

Unknown.

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.

CVE

ID Summary CVSS
CVE-2021-34527 Windows Print Spooler Remote Code Execution Vulnerability 9.0

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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