Boot or Logon Autostart Execution: Port Monitors

Adversaries may use port monitors to run an attacker 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. [1] This DLL can be located in C:\Windows\System32 and will be loaded by the print spooler service, spoolsv.exe, on boot. The spoolsv.exe process also runs under SYSTEM level permissions. [2] Alternatively, an arbitrary DLL can be loaded if permissions allow writing a fully-qualified pathname for that DLL to HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors.

The Registry key contains entries for the following:

  • Local Port
  • Standard TCP/IP Port
  • USB Monitor
  • WSD Port

Adversaries can use this technique to load malicious code at startup that will persist on system reboot and execute as SYSTEM.

ID: T1547.010
Sub-technique of:  T1547
Tactics: Persistence, Privilege Escalation
Platforms: Windows
Permissions Required: Administrator, SYSTEM
Effective Permissions: SYSTEM
Data Sources: File: File Creation, Module: Module Load, Process: OS API Execution, Windows Registry: Windows Registry Key Modification
Contributors: Stefan Kanthak; Travis Smith, Tripwire
Version: 1.0
Created: 24 January 2020
Last Modified: 24 January 2020


This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.


Monitor process API calls to AddMonitor.[1] Monitor DLLs that are loaded by spoolsv.exe for DLLs that are abnormal. New DLLs written to the System32 directory that do not correlate with known good software or patching may be suspicious.

Monitor Registry writes to HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors. Run the Autoruns utility, which checks for this Registry key as a persistence mechanism [3]