Detect Port Monitors in Splunk
Adversaries may use port monitors to run an adversary-supplied DLL during system boot for persistence or privilege escalation. A port monitor can be set through the AddMonitor API call to set a DLL to be loaded at startup. This DLL can be located in C:\Windows\System32 and will be loaded and run by the print spooler service, spoolsv.exe, under SYSTEM level permissions on boot. Alternatively, an arbitrary DLL can be loaded if permissions allow writing a fully-qualified pathname for that DLL to the Driver value of an existing or new arbitrarily named subkey of HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors.
MITRE ATT&CK
- Tactic
- Persistence Privilege Escalation
- Technique
- T1547 Boot or Logon Autostart Execution
- Sub-technique
- T1547.010 Port Monitors
- Canonical reference
- https://attack.mitre.org/techniques/T1547/010/
SPL Detection Query
index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=13
TargetObject="HKLM\\SYSTEM\\CurrentControlSet\\Control\\Print\\Monitors\\*"
| eval is_driver_value=if(match(TargetObject, "Driver$"), 1, 0)
| eval is_dll=if(match(Details, "\.dll"), 1, 0)
| eval monitor_name=replace(TargetObject, ".*Print\\Monitors\\\\.+?\\(.*)", "\1")
| table _time, host, User, Image, TargetObject, Details, is_driver_value, is_dll, monitor_name
| 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, ParentCommandLine
| sort - _time]
| append
[search index=wineventlog sourcetype="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=11
TargetFilename="C:\\Windows\\System32\\*.dll"
NOT (Image="*\\TiWorker.exe" OR Image="*\\TrustedInstaller.exe" OR Image="*\\msiexec.exe")
| table _time, host, User, Image, TargetFilename
| sort - _time]Detects port monitor persistence using three Sysmon event types: (1) Event ID 13 (Registry Value Set) for modifications to the Print\Monitors registry key, indicating new port monitor DLL registration; (2) Event ID 1 (Process Creation) for unexpected child processes spawned by spoolsv.exe; and (3) Event ID 11 (File Create) for new DLLs written to System32 by non-standard processes. Results are combined via append for unified analyst review.
Data Sources
Required Sourcetypes
False Positives & Tuning
- Legitimate printer driver installations by IT administrators using vendor-provided installers that register port monitors via AddMonitor API
- Print management software (PaperCut, PrinterLogic) installing custom port monitors for enterprise print tracking
- PDF virtual printer utilities (Adobe PDF, CutePDF) that register port monitors during installation
- Windows Update deploying printer driver packages that modify Print\Monitors registry keys
Other platforms for T1547.010
Testing Methodology
Validate this detection against 3 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 1Port Monitor Registry Key Creation
Expected signal: Sysmon Event ID 13: Registry Value Set with TargetObject=HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors\TestMonitor_df00tech\Driver, Details=localspl.dll, Image=reg.exe. Security Event ID 4688 with CommandLine containing reg add.
- Test 2DLL Drop in System32 Simulating Port Monitor Payload
Expected signal: Sysmon Event ID 11: File Create with TargetFilename=C:\Windows\System32\df00tech_test_monitor.dll, Image=cmd.exe. DeviceFileEvents with ActionType=FileCreated, FolderPath starting with C:\Windows\System32.
- Test 3PowerShell AddMonitor API Simulation via Registry
Expected signal: Sysmon Event ID 12: Registry Key Created for PSTestMonitor subkey. Sysmon Event ID 13: Registry Value Set for Driver value. Sysmon Event ID 1: Process Create for powershell.exe with the full command line. PowerShell ScriptBlock Log Event ID 4104 with the registry manipulation commands.
References (7)
- https://attack.mitre.org/techniques/T1547/010/
- https://www.defcon.org/images/defcon-22/dc-22-presentations/Bloxham/DEFCON-22-Brady-Bloxham-Windows-API-Abuse-UPDATED.pdf
- https://learn.microsoft.com/en-us/windows/win32/printdocs/addmonitor
- https://technet.microsoft.com/en-us/sysinternals/bb963902
- https://learn.microsoft.com/en-us/defender-endpoint/advanced-hunting-deviceregistryevents-table
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1547.010/T1547.010.md
- https://github.com/SigmaHQ/sigma/tree/master/rules/windows/registry/registry_set
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