Detect Port Monitors in Microsoft Sentinel
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/
KQL Detection Query
let MonitorRegPath = @"HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors";
DeviceRegistryEvents
| where Timestamp > ago(24h)
| where RegistryKey has "Control\\Print\\Monitors"
| where ActionType in ("RegistryValueSet", "RegistryKeyCreated")
| extend RegistryValueName = tostring(RegistryValueData)
| extend SuspiciousDLL = RegistryValueData endswith ".dll"
| project Timestamp, DeviceName, AccountName, ActionType, RegistryKey, RegistryValueName, RegistryValueData, InitiatingProcessFileName, InitiatingProcessCommandLine, SuspiciousDLL
| sort by Timestamp desc;
DeviceProcessEvents
| where Timestamp > ago(24h)
| where InitiatingProcessFileName =~ "spoolsv.exe"
| where FileName !in~ ("splwow64.exe", "PrintIsolationHost.exe", "printfilterpipelinesvc.exe")
| project Timestamp, DeviceName, AccountName, FileName, ProcessCommandLine, FolderPath, InitiatingProcessFileName
| sort by Timestamp desc;
DeviceFileEvents
| where Timestamp > ago(24h)
| where FolderPath startswith @"C:\Windows\System32"
| where FileName endswith ".dll"
| where InitiatingProcessFileName !in~ ("TiWorker.exe", "TrustedInstaller.exe", "msiexec.exe", "svchost.exe")
| where ActionType == "FileCreated"
| project Timestamp, DeviceName, AccountName, FileName, FolderPath, InitiatingProcessFileName, InitiatingProcessCommandLine
| sort by Timestamp descDetects port monitor persistence via three signals: (1) Registry modifications to the Print\Monitors key indicating a new port monitor DLL registration, (2) spoolsv.exe spawning unexpected child processes that may indicate a malicious port monitor DLL executing code, and (3) new DLL files dropped into C:\Windows\System32 by non-standard processes. Uses DeviceRegistryEvents, DeviceProcessEvents, and DeviceFileEvents tables from Microsoft Defender for Endpoint.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate printer driver installations by IT administrators using vendor-provided installers (HP, Canon, Lexmark, etc.) that register port monitors through AddMonitor API
- Print management software (PaperCut, PrinterLogic, Pharos) that installs custom port monitors for print job tracking and accounting
- PDF printer utilities (Adobe PDF, Microsoft Print to PDF, CutePDF) that register virtual port monitors during installation
- Windows Update or WSUS deploying printer driver updates that modify the Print\Monitors registry key
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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