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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

Annotations

ATT&CK

ATT&CK

ID Technique Tactic
T1547.012 Print Processors Persistence, Privilege Escalation
T1547 Boot or Logon Autostart Execution Persistence, Privilege Escalation
Kill Chain Phase
  • Exploitation
NIST
CIS20
CVE
ID Summary CVSS
CVE-2021-34527 Windows Print Spooler Remote Code Execution Vulnerability 9.0 


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`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`

Macros

The SPL above uses the following Macros:

:information_source: spoolsv_writing_a_dll_-_sysmon_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

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

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.

Known False Positives

Limited false positives. Filter as needed.

Associated Analytic Story

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.

:information_source: The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

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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