Detect Print Processors in Splunk
Adversaries may abuse print processors to run malicious DLLs during system boot for persistence and/or privilege escalation. Print processors are DLLs that are loaded by the print spooler service, spoolsv.exe, during boot. Adversaries may abuse the print spooler service by adding print processors that load malicious DLLs at startup. A print processor can be installed through the AddPrintProcessor API call with an account that has SeLoadDriverPrivilege enabled. Alternatively, a print processor can be registered to the print spooler service by adding a Registry key under HKLM\SYSTEM\CurrentControlSet\Control\Print\Environments\Windows x64\Print Processors with a Driver value pointing to the malicious DLL. The print spooler service runs under SYSTEM level permissions, therefore print processors installed by an adversary may run under elevated privileges.
MITRE ATT&CK
- Tactic
- Persistence Privilege Escalation
- Technique
- T1547 Boot or Logon Autostart Execution
- Sub-technique
- T1547.012 Print Processors
- Canonical reference
- https://attack.mitre.org/techniques/T1547/012/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=13
TargetObject="HKLM\\SYSTEM\\*Control\\Print\\Environments\\*Print Processors*"
| eval is_driver_value=if(match(TargetObject, "\\Driver$"), 1, 0)
| eval is_known=if(match(Details, "(winprint|filterpipelineprintproc)\.dll"), 1, 0)
| eval processor_name=replace(TargetObject, ".*Print Processors\\\\(.+?)\\\\.*", "\1")
| table _time, host, User, Image, TargetObject, Details, is_driver_value, is_known, processor_name
| sort - _time
| append
[search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=11
TargetFilename="*\\spool\\prtprocs\\*"
TargetFilename="*.dll"
| table _time, host, User, Image, TargetFilename
| sort - _time]
| append
[search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1
ParentImage="*\\spoolsv.exe"
NOT (Image="*\\splwow64.exe" OR Image="*\\PrintIsolationHost.exe" OR Image="*\\printfilterpipelinesvc.exe")
| table _time, host, User, Image, CommandLine, ParentImage
| sort - _time]Detects print processor persistence using three Sysmon event types: (1) Event ID 13 for registry modifications to Print\Environments\*\Print Processors indicating new print processor registration, (2) Event ID 11 for DLL file creation in the spool\prtprocs directory, and (3) Event ID 1 for unexpected child processes of spoolsv.exe. The processor_name field is extracted from the registry path for quick identification of the registered print processor.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate printer driver installations from vendors that register custom print processors during setup
- Enterprise print management solutions (PaperCut, Pharos) deploying custom print processors
- Windows Update deploying printer driver updates that include print processor DLLs
- IT administrators installing print processors on print servers using vendor-provided tools
Other platforms for T1547.012
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Print Processor Registry Key Creation
Expected signal: Sysmon Event ID 12: Registry Key Created for TestProcessor_df00tech subkey. Sysmon Event ID 13: Registry Value Set for Driver value with Details=winprint.dll. Security Event ID 4688 with CommandLine containing reg add and the full registry path.
- Test 2DLL Drop in Print Processor Directory
Expected signal: Sysmon Event ID 11: File Create with TargetFilename=C:\Windows\System32\spool\prtprocs\x64\df00tech_test_printproc.dll, Image=cmd.exe. DeviceFileEvents with ActionType=FileCreated in the prtprocs directory.
- Test 3PowerShell Print Processor Registration (Earth Lusca Style)
Expected signal: Sysmon Event ID 1: Process Create for powershell.exe with full command line. Sysmon Event ID 12: Registry Key Created. Sysmon Event ID 13: Registry Value Set. PowerShell ScriptBlock Log Event ID 4104 capturing the registry manipulation.
- Test 4Enumerate Existing Print Processors for Reconnaissance
Expected signal: Sysmon Event ID 1: Process Create for reg.exe with CommandLine containing reg query and Print Processors path. Security Event ID 4688 with same details. No registry modification events (read-only operation).
References (7)
- https://attack.mitre.org/techniques/T1547/012/
- https://docs.microsoft.com/en-us/windows/win32/printdocs/addprintprocessor
- https://learn.microsoft.com/windows-hardware/drivers/print/introduction-to-print-processors
- https://www.welivesecurity.com/2020/05/21/no-game-over-winnti-group/
- https://www.trendmicro.com/en_us/research/22/a/earth-lusca-employs-sophisticated-infrastructure-varied-tools-and-techniques.html
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1547.012/T1547.012.md
- https://github.com/SigmaHQ/sigma/tree/master/rules/windows/registry/registry_set
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