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Monitoring with PowerShell: AD KRBTGT & making your own canaries

I decided this time I’m gonna be combining two small blogs, because they’re both pretty small and easy. Both are somewhat security oriented. The first part of the blog we will tackle monitoring the KRBTGT password. This needs to be reset on a regular schedule to ensure bad actors can’t abuse it.

The second part we’ll focus on creating our own ‘Canary’ files. These files can be used for a lot of things but the most common is to detect if ransomware has touched them in someway or the other. So, lets get started!

Monitoring KRBTGT Password age

So it’s actually straight forward to monitor the KRBTGT account, as it’s just a AD account. We’ll monitor this by grabbing the PasswordLastSet Attributes from the Active Directory. If you want to automatically resolve this, I’d strongly suggest to look at the script in this Github.

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$Days = (Get-Date).AddDays(-31)
$Account = Get-AdUser krbtgt -property passwordlastset
$Setdate = if($Account.PasswordLastSet -gt $Days){ "Healthy - Password set date $($Account.Passwordlastset)" } else {" Unhealthy - Password set date $($Account.Passwordlastset)" }

You can change the amount of days to what you are comfortable with. I believe the documentation doesn’t have a strong suggestion in how much you should, but as this is a completely automated solution we perform this on a monthly basis.

Creating and monitoring file canaries

So, canaries are files that you place on strategic locations on a machine to check if the files aren’t being touched, corrupted, or encrypted in any way. Primarily they are used to prevent a full encryption of a computer and minimize data loss and lateral movement.

So with this script, we create canaries in a couple of locations;

  • The My Documents folder of each user
  • The Desktop Folder of each user
  • The root of each drive on the machine

You’ll also be able to create them in locations you want by adding to the $CreateLocations variable. We create the files as hidden, so users should not see the file, the file name will be canaryfile.pdf, even though it’s just a simple text file.

So the script creates a file in each location, and immediately starts alerting on two properties; If the file has been edited in the past hour, and if the file contains the correct string. I’d advice to apply the monitoring to the device, wait an hour, and then actually start alerting on it or reacting.

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$CreateLocations = @('AllDesktops', 'AllDocuments', 'AllDrives', 'C:\temp')
$FileContent = "This file is a special file created by your managed services provider. For more information contact the IT Support desk."

foreach ($Locations in $CreateLocations) {
    $AllLocations = switch ($Locations) {
"AllDesktops" { (Get-ChildItem "C:\Users" -Recurse -Force -filter 'Desktop' -Depth 3).FullName }
"AllDocuments" { (Get-ChildItem "C:\Users" -Recurse -Force -Filter 'Documents' -Depth 3).fullname }
"AllDrives" { ([System.IO.DriveInfo]::getdrives() | Where-Object { $_.DriveType -eq 'Fixed' }).Name }
        default { $Locations }
    }
  $CanaryStatus = foreach ($Location in $AllLocations) {
        if ((test-path "$Location\CanaryFile.pdf") -eq $false) {
            $File = New-Item $Location -Name "CanaryFile.pdf" -Value $FileContent
            $file.Attributes = 'hidden'
        }
        else {
            $ExistingFile = get-item "$Location\CanaryFile.pdf" -Force
if ($ExistingFile.LastWriteTime -gt (get-date).AddHours(-1)) { "$Location\CanaryFile.pdf is unhealthy. The LastWriteTime was $($ExistingFile.LastWriteTime)" }
$ExistingFileContents = get-content $ExistingFile -Force
            if ($ExistingFileContents -ne $FileContent) { "$Location\CanaryFile.pdf is unhealthy. The contents do not match. This is a sign the file has most likely been encrypted" }
}
}
}
if(!$CanaryStatus){
$CanaryStatus = "Healthy"
}

$CanaryStatus

If you feel confident enough about this, you could set up some self-healing like disabling network access, or shutting the device down before the machine is completely encrypted. And that’s it. As always, Happy PowerShelling!

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