Monitoring with PowerShell: Monitoring log on of specific users.

Hi Guys, This’ll be the last blog before I go on holidays, So enjoy it and see you all in two weeks.

This time we’re going to talk about montoring the logon of specific users. We use named accounts for all our engineers and want to alert if another account that is unnamed has been logged onto in an interactive session. To do this, we’ll use the WMI instrumentation Win32_LoggedOnUser.

Just as an extra disclaimer: Please remember that in co-managed or environments that belong to others you won’t be able to always perform all best practices. This script will mostly be used to monitor those messy environments, and give you a little bit more sense of extra security. My personal advice would always be to disable accounts that are no longer allowed to login, use managed service accounts without interactive permissions for services, and delete accounts of ex-employees directly after the leave the company.

The script

Let’s get started on the script. First we’ll have to define which accounts we do not want to have in interactive sessions:

$ForbiddenList = @("Cyberdrain","cyber","migration","administrator","admin","service-QuickBooks*","svc-QB*","ExEmployee1")

So in this list, we name all accounts that are forbidden. You can add any user you would like to this list, We manage a lot of servers, and sometimes after an engineer leaves our company servers are still logged in with the user that we’ve disabled/deleted. With this script we also monitor those situations and log out the deleted user from all servers.

The next step is getting a list of the active users and comparing them:

$ActiveUsers = (Get-CimInstance Win32_LoggedOnUser).antecedent | Select-Object -Unique | Where-Object {$_.name -in $ForbiddenList}

So here we get all users that are currently logged on to the machine, and compare them to our forbidden list, our $activeUsers variable only gets filled if there is a match in the list.

if(!$ActiveUsers){$ActiveUsers = 'false'}

And this last line says that if $ActiveUsers is empty, so no users have been found that are logged in and in our list, it will say “false”. The complete script is just 3 lines and can be found below.

$ForbiddenList = @("Cyberdrain","cyber","migration","administrator","admin","service-QuickBooks*","svc-QB*","ExEmployee1")
$ActiveUsers = (Get-CimInstance Win32_LoggedOnUser).antecedent | Select-Object -Unique | Where-Object {$_.name -in $ForbiddenList}
if(!$ActiveUsers){$ActiveUsers = 'false'}

And that’s it. Some monitoring for situations you do not want to end up in. Remember to always follow security best practices first. Only use these scripts as an early warning system that someone, somewhere has made a mistake. 🙂 As always, Happy PowerShelling!

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Kelvin Tegelaar

I am a Microsoft Certified System Engineer working as the CTO of the Managed Services Provider Lime Networks B.V. in the Netherlands. I mostly enjoy automating business processes by deploying PowerShell solutions, but just have a large passion for Microsoft Technology in general.

If you want to contact me directly you can find me on twitter here, or via email: Kelvin {at} limenetworks.nl
Kelvin Tegelaar
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