Port Monitors
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. The Registry key contains entries for the following: * Local Port * Standard TCP/IP Port * USB Monitor * WSD Port
Open detection, hunting, mitigation, and evidence workspace
Detection logic
Monitor process API calls to AddMonitor. 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, paying particular attention to changes in the "Driver" subkey. Run the Autoruns utility, which checks for this Registry key as a persistence mechanism.