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Boot or Logon Autostart Execution: Print Processors
Other sub-techniques of Boot or Logon Autostart Execution (12)
Adversaries may abuse print processors to run malicious DLLs during system boot for persistence and/or privilege escalation. Print processors are DLLs that are loaded by the print spooler service, spoolsv.exe, during boot.
Adversaries may abuse the print spooler service by adding print processors that load malicious DLLs at startup. A print processor can be installed through the AddPrintProcessor
API call with an account that has SeLoadDriverPrivilege
enabled. Alternatively, a print processor can be registered to the print spooler service by adding the HKLM\SYSTEM\[CurrentControlSet or ControlSet001]\Control\Print\Environments\[Windows architecture: e.g., Windows x64]\Print Processors\[user defined]\Driver
Registry key that points to the DLL. For the print processor to be correctly installed, it must be located in the system print-processor directory that can be found with the GetPrintProcessorDirectory
API call.[1] After the print processors are installed, the print spooler service, which starts during boot, must be restarted in order for them to run.[2] The print spooler service runs under SYSTEM level permissions, therefore print processors installed by an adversary may run under elevated privileges.
Procedure Examples
Name | Description |
---|---|
PipeMon |
The PipeMon installer has modified the Registry key |
Mitigations
Mitigation | Description |
---|---|
User Account Management |
Limit user accounts that can load or unload device drivers by disabling |
Detection
Monitor process API calls to AddPrintProcessor
and GetPrintProcessorDirectory
. New print processor DLLs are written to the print processor directory. Also monitor Registry writes to HKLM\SYSTEM\ControlSet001\Control\Print\Environments\[Windows architecture]\Print Processors\[user defined]\Driver
or HKLM\SYSTEM\CurrentControlSet\Control\Print\Environments\[Windows architecture]\Print Processors\[user defined]\Driver
as they pertain to print processor installations.
Monitor for abnormal DLLs that are loaded by spoolsv.exe. Print processors that do not correlate with known good software or patching may be suspicious.