Hello again fellow geeks!
Recently I was giving a customer an overview of Azure Managed Identities and came across an interesting find while building a demo environment. If you’re unfamiliar with managed identities, check out my prior series for an overview. Long story short, managed identities provide a solution for non-human identities where you don’t have to worry about storing, securing, and rotating the credentials. For those of you coming from AWS, managed identities are very similar to AWS Roles. They come in two flavors, user-assigned and system-assigned. For the purposes of this post, I’ll be focusing on system-assigned.
Under the hood, a managed identity is essentially a service principal with some orchestration on top of it. Interestingly enough, there are a number of different service principal types. Running the command below will spit back the different types of service principals that exist in your Azure AD tenant.
az ad sp list --query='.servicePrincipalType' --all | sort | uniq
If you’re interested in seeing the service principals associated with managed identities in your Azure AD tenant, you can run the command below.
az ad sp list --query="[?servicePrincipalType=='ManagedIdentity']" --all
Managed identities include a property called alternativeNames which is an array. In my testing I observed two values within this array. The first value is “isExplicit=True” or “isExplicit=False” which is set to True for user-assigned managed identities and False when it’s a system-assigned managed identity. If you want to see all system-assigned managed identities for example, you can run the command below.
az ad sp list --query="[?servicePrincipalType=='ManagedIdentity' && alternativeNames[?contains(@,'isExplicit=False')]]" --all
The other value in this array is the resource id of the managed identity in the case of a user-assigned managed identity. With a system-assigned managed identity this is the resource id of the Azure resource the system-assigned managed identity is associated with.
So why does any of this matter? Before we get to that, let’s cover the major selling point of a system-assigned managed identity when compared to a user-assigned managed identity. With a system-assigned managed identity, the managed identity (and its service principal) share the lifecycle of the resource. This means that if you delete the resource, the service principal is cleaned up… well most of the time anyway.
Sometimes this cleanup process doesn’t happen and you’re left with orphaned service principals in your directory. The most annoying part is you can’t delete these service principals (I’ve tried everything including calls direct to the ARM API) and the only way to get them removed is to open a support ticket. Now there isn’t a ton of risk I can think of with having these orphaned service principals left in your tenant since I’m not aware of any means to access the credential associated with it. Without the credential no one can authenticate as it. Assuming the RBAC permissions are cleaned up, it’s not really authorized to do anything within Azure either. However, beyond dirtying up your directory, it’s an identity with a credential that shouldn’t be there anymore.
I wanted an easy way to identify these orphaned system-assigned managed identities so I could submit a support ticket and get it cleaned up before it started cluttering up my demonstration tenant. This afternoon I wrote a really ugly bash script to do exactly that. The script uses some of the az cli commands I’ve listed above to identify all the system-assigned managed identities and then uses az cli to determine if the resource exists. If the resource doesn’t exist, it logs the displayName property of the system-assigned managed identity to a text file. Quick and dirty, but does the job.
Interestingly enough, I had a few peers run the script on their tenants and they all had some of these orphaned system-assigned managed identities, so it seems like this problem isn’t restricted to my tenants. Again, I personally can’t think of a risk of these identities remaining in the directory, but it does point to an issue with the lifecycle management processes Microsoft is using in the backend.
Well folks that’s it! Have a great night!