Detect Port Monitors in Elastic Security
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/
Elastic Detection Query
sequence by host.name with maxspan=5m
[registry where registry.path : "HKLM\\SYSTEM\\CurrentControlSet\\Control\\Print\\Monitors\\*" and event.action in ("registry_value_set", "registry_key_created")]
[file where file.path : "C:\\Windows\\System32\\*.dll" and event.action == "creation" and not process.name : ("TiWorker.exe", "TrustedInstaller.exe", "msiexec.exe", "svchost.exe")]
sequence
[registry where registry.path : "HKLM\\SYSTEM\\CurrentControlSet\\Control\\Print\\Monitors\\*" and registry.data.strings : "*.dll"]
[process where process.parent.name : "spoolsv.exe" and not process.name : ("splwow64.exe", "PrintIsolationHost.exe", "printfilterpipelinesvc.exe")]Detects T1547.010 Port Monitor persistence via Print Monitors registry key modification, suspicious DLL drops in System32, and unusual child processes spawned by spoolsv.exe. Uses EQL sequences to correlate registry modifications with subsequent file creation or spooler child processes.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate printer driver installations that register custom port monitors (e.g., HP, Canon, Xerox printer software)
- Print management software updates that modify the Print Monitors registry key during upgrades
- Windows Update or system components like TrustedInstaller deploying spooler-related DLLs to System32
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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