Null

Null Dynamic Membership Rules in Azure Active Directory

Azure Active Directory has the ability to create Security Groups with Dynamic membership. This is great if you can apply logic to a group, as members will fall in and out of scope without any work required.

Microsoft have a great writeup on how it all works and how to create rules, however I’ve run into a scenario not covered in the documentation.

If you create a Dynamic membership rule and want to include only attributes that have no value, the term ‘null’ works fine. You can create your group or modify the rule without issue.
However, if your binary operator (the equals part in the example above) is set to ‘not’, it won’t work.
The use case I had for ‘not null’ was to have a group of users which only had employee numbers, which was an easy way of filtering out test accounts, service accounts and so on.

You’ll get this error:

Failed to create group

Failed to create group ‘groupname’. Dynamic membership rule validation error: Invalid operands found for operator.Invalid operands found for operator -not

The way to fix this is to go into the ‘Advanced Rule’ option and change the term ‘null’ to ‘$null’

Note that you can’t do this from the simple rule view, changing ‘null’ to ‘$null’ there results in the code looking like this:

(user.extensionAttribute1 -eq “$null”)

 

Where it should look like this, without the quotes:

(user.extensionAttribute1 -eq $null)

A simple fix, but something that’s not documented on the support page. Hope this helps anyone who runs into the same problem.

Null and Not Null with PowerShell

Finding out if an object has a null (i.e. blank) value or not isn’t a difficult task to do.

Consider this scenario – you’ve found a bunch of old disabled accounts that someone forgot to remove the ‘Manager’ field. Finding accounts that have another field that would be populated for a current employee but blank for a departed would be a reasonable way of finding the problem accounts, then you could null the ‘Manager field. (note – you could just refine your search to disabled accounts but that’s not as fun).

To find all Active Directory users that have a blank ‘Department’ field is easily done with this command:

get-aduser -filter * -properties department | where department -eq $null

Then, showing the users that don’t have a blank ‘Department’ field is a slight change. You can’t use !$null (!=not), but you can use -ne (not equals)

get-aduser -filter * -properties department | where department -ne $null

You can also check for users that have a manger by switching ‘department’ to ‘manager’:

get-aduser -filter * -properties maanger | where manager -ne $null

Easy. Adding in a second ‘where’ statement so we can get results of users that have a manager, but no department means we have to add in a few extra characters to make PowerShell happy:

get-aduser -filter * -properties department,manager | where {($_.department -eq $null) -and ($_.manager -ne $null)}

The results can be a bit hard to read, so piping (|) to a select command will just show us the results of each user we want to see:

get-aduser -filter * -properties department,manager | where {($_.department -eq $null) -and ($_.manager -ne $null)} | select name

Finally, to blank the ‘manager’ field, we can swap the ‘select name’ command with this:

get-aduser -filter * -properties department,manager | where {($_.department -eq $null) -and ($_.manager -ne $null)} |  set-aduser -manager $null

You can then go back to a previous command to confirm you get no results. As always, check your data first before blanking out a bunch of user’s values!

Update

As @mickesunkan pointed out, the above isn’t the most efficient way to do searches. I’m sure I’ve mentioned this before, but I’m not always going to write the cleanest, quickest way of doing something. For a once off tasks this really doesn’t matter. For a daily task it starts to matter – not really by itself, but if you keep making more and more inefficient scripts, you’re putting extra unnecessary load on your environment with lots of LDAP lookups.

Above, I’m just getting ALL AD users. You could use a better filter and narrow down to a certain OU. You could also put part of your ‘where’ command into the filter, such as this:

get-aduser -properties manager,department -filter {department -notlike “*”}

This doesn’t work for the ‘Manager’ field though, you’ll see this error:

get-aduser : Operator(s): The following: ”Eq’, ‘Ne” are the only operator(s) supported for searching on extended attribute: ‘Manager’.

I couldn’t work out a way of putting the $null value as part of the filter, but if you do – please share :)

 

@mickesunkan also wrote this github code showing a few differnet ways to do this search, and which way is most efficient. Thanks Micke!