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T1547.012 - Print Processors

T1547.012 - Print Processors#

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.(Citation: Microsoft Intro Print Processors)

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 malicious print processor to be correctly installed, the payload must be located in the dedicated system print-processor directory, that can be found with the GetPrintProcessorDirectory API call, or referenced via a relative path from this directory.(Citation: Microsoft AddPrintProcessor May 2018) After the print processors are installed, the print spooler service, which starts during boot, must be restarted in order for them to run.(Citation: ESET PipeMon May 2020)

The print spooler service runs under SYSTEM level permissions, therefore print processors installed by an adversary may run under elevated privileges.

Atomic Tests#

Atomic Test #1 - Print ProcessorsEstablishes persistence by creating a new print processor registry key under HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Print\Environments\Windows x64\Print Processors.#

The new print processor will point to a DLL which will be loaded by the spooler service after a reboot. The DLL will then create the file AtomicTest.txt in C:\Users\Public\ as validation that the test is successful.

Note: The test assumes a x64 Windows operating system.

The payload source code is based on a blog post by stmxcsr: https://stmxcsr.com/persistence/print-processor.html Supported Platforms: windows Elevation Required (e.g. root or admin)#### Attack Commands: Run with powershell

if( $(get-service -Name spooler).StartType -eq "Disabled") {Set-Service -Name "spooler" -StartupType Automatic}
net stop spooler
Copy-Item "$PathToAtomicsFolder\T1547.012\bin\AtomicTest.dll" C:\Windows\System32\spool\prtprocs\x64\AtomicTest.dll
reg add "HKLM\SYSTEM\CurrentControlSet\Control\Print\Environments\Windows x64\Print Processors\AtomicRedTeam" /v "Driver" /d "AtomicTest.dll" /t REG_SZ /f
net start spooler
if(0){
  Restart-Computer
}

Invoke-AtomicTest T1547.012 -TestNumbers 1

Cleanup:#

net stop spooler
rm -force C:\Windows\System32\spool\prtprocs\x64\AtomicTest.dll -ErrorAction SilentlyContinue
rm -force C:\Users\Public\AtomicTest.txt -ErrorAction SilentlyContinue
remove-item "HKLM:\SYSTEM\CurrentControlSet\Control\Print\Environments\Windows x64\Print Processors\AtomicRedTeam" -Force -ErrorAction SilentlyContinue
net start spooler

Invoke-AtomicTest T1547.012 -TestNumbers 1 -Cleanup

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.

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