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 <code>AddMonitor</code> API call to set a DLL to be loaded at startup.(Citation: AddMonitor) This DLL can be located in <code>C:\Windows\System32</code> and will be loaded and run by the print spooler service, `spoolsv.exe`, under SYSTEM level permissions on boot.(Citation: Bloxham) 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 <code>HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors</code>. The Registry key contains entries for the following: * Local Port * Standard TCP/IP Port * USB Monitor * WSD Port
Detects suspicious registry modifications under `HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors\*\Driver`, DLL loads by `spoolsv.exe` of non-standard or unsigned modules, and abnormal usage of the `AddMonitor` API by non-installation processes. This pattern often indicates an attempt to persist a malicious DLL via the print monitor mechanism, particularly when correlated with creation of files in `C:\Windows\System32` not tied to known patches or installations.