How can I create a new scheduled Task when running script as "local system"?

Question

I am not able to create a scheduled task under latest Win11 when running a Powershell script as "local system". It gets me an error "New-ScheduledTaskTrigger : Cannot connect to CIM server. Access to a CIM resource was not available to the client."

Here is the code to replicate the issue. How can it be solved?:

#requires -runasadmin

# this works as admin:
$trigger = New-ScheduledTaskTrigger -Once -At (get-date)

# switching to local system:
$id = [System.Security.Principal.WindowsIdentity]
if (-not $id::GetCurrent().IsSystem) {
    $advApi = add-type '
        [DllImport("kernel32.dll")]
        public static extern IntPtr OpenProcess(UInt32 Access, bool inherit, UInt32 pid);
        [DllImport("advapi32.dll")]
        public static extern bool OpenProcessToken(IntPtr Thread, UInt32 Access, out IntPtr Handle);
        [DllImport("advapi32.dll")]
        public extern static bool DuplicateToken(IntPtr Handle, int Level, out IntPtr Copy);
        [DllImport("advapi32.dll")]
        public static extern bool SetThreadToken(IntPtr Thread, IntPtr Token);
    ' -Name advApi -PassThru
    $service = Get-WmiObject win32_service | where { $_.name -eq 'SamSs'}
    $sysHandle = $advApi::OpenProcess(0x1000, $false, $service.processID)
    $otherToken = [IntPtr]0
    $myToken    = [IntPtr]0
    $null = $advApi::OpenProcessToken($sysHandle, 6, [ref]$otherToken)
    $null = $advApi::DuplicateToken($otherToken, 2, [ref]$myToken)
    $null = $advApi::SetThreadToken(0, $myToken)
}
if ($id::GetCurrent().IsSystem) {
    "now we have local system rights for this process."
}

# this is failing now:
$trigger = New-ScheduledTaskTrigger -Once -At (get-date)

# back to admin:
$advApi::SetThreadToken(0, 0)

# this works again:
$trigger = New-ScheduledTaskTrigger -Once -At (get-date)

Answer 1

After testing many different approaches, I came to the conclusion that I cannot use the classic PS-command for creating a scheduled task in a reliable way when the main process runs as "Local system". Therefore I decided to go with this approach which works very reliable on any Windows-version:

# Create a new scheduler object:
$scheduler = New-Object -ComObject Schedule.Service
$scheduler.Connect()

# Create a new task:
$task = $scheduler.NewTask(0)
$task.Principal.UserId = "NT AUTHORITY\SYSTEM"
$task.Principal.RunLevel = 1

# Create an action to start the executable:
$action = $task.Actions.Create(0)
$action.Path = "notepad.exe"
$action.WorkingDirectory = "C:\Windows"

# Create a trigger to run the task once during registration:
$trigger = $task.Triggers.Create(7) 

# Register and trigger the task in the specified folder:
$root = $scheduler.GetFolder("\")
try {
    $folder = $root.GetFolder('Systrack')
} catch {
    $folder = $root.CreateFolder('Systrack')
}
$entry = $folder.RegisterTaskDefinition("RunNotepadTask", $task, 6, $null, $null, 3)

# Do the clean-up:
$folder.DeleteTask("RunNotepadTask", 0)
$root.DeleteFolder("Systrack",0)