Hi everyone, it’s Gershon, back again with a follow up to my last blog where we were able to track changes to sensitive groups with Advanced Hunting in Microsoft 365 Defender. One of the questions I had from a customer after they read through the blog was “how can we be alerted directly when a group has been added to a sensitive group?”. This is a great question to focus on, as this scenario should not be commonplace in an established environment, as nested group memberships in sensitive groups should not be something that changes after initial set up. Additionally, adding a group to another group is a quick and easy way to add users to a sensitive group and making sure it’s highlighted quickly could stop an attacker from gaining persistence.
We will start by assuming that all the steps from my previous blog have been completed, where we can see all the changes to the groups we defined as sensitive.
In this blog, we will take things further by:
Starting with the query from the last blog as a starting point, we will make a few changes that focuses on activities that occur when adding a group to a sensitive group.
In the last line, we will also add the “AccountSid” column as this can be used in the custom detection policy, covered later in this entry.
let SensitiveGroupName = pack_array( // Declare Sensitive Group names. Add any groups that you manually tagged as sensitive 'Account Operators', 'Administrators', 'Domain Admins', 'Backup Operators', 'Domain Controllers', 'Enterprise Admins', 'Enterprise Read-only Domain Controllers', 'Group Policy Creator Owners', 'Incoming Forest Trust Builders', 'Microsoft Exchange Servers', 'Network Configuration Operators', 'Print Operators', 'Read-only Domain Controllers', 'Replicator', 'Schema Admins', 'Server Operators' ); IdentityDirectoryEvents | where Application == "Active Directory" | where ActionType == "Group Membership changed" | where DestinationDeviceName != "" // Exclude activities coming from AD Sync changes | extend GroupModified = tostring(parse_json(AdditionalFields).["TO.GROUP"]) // Extracts the group name if action is add enity to a group. | where GroupModified in~ (SensitiveGroupName) | extend GroupAdded = tostring(parse_json(AdditionalFields).["TARGET_OBJECT.GROUP"]) | where GroupAdded != "" // only show activites that are group being added to another group | project Timestamp, GroupModified, GroupAdded, Actor=AccountName, Actor_UPN=AccountUpn, ActorSid=AccountSid, AccountSid, ActorDomain=AccountDomain, DC=DestinationDeviceName, ReportId, AdditionalFields | sort by Timestamp desc
As you can see here, the results only show groups that have been added to a sensitive group.
If you don’t see any results when you run your query, you can either change the time range or add a group to one of the sensitive groups listed in the array at the beginning of the query.
After you are comfortable with the query, we can now create the custom detection rule.
The customer I was working with selected High for Severity as this is not something that should happen often, if at all, in their environment.
For the MITRE techniques the customer chose:
Now we want to test that the new rule is working as expected.
In Active Directory, create a new group and add this group to a sensitive group. In my lab I created a group named “TestGroupforBlog” and added it as a member of “Domain Admins.”
There are different ways that we can search for the alert. Here is one way:
To see the activities that triggered this alert click Query results (1). You can see the sensitive group that was modified (2), the group that was added to the sensitive group (3) and the user who made this change (4).
If this is an approved legitimate change, we would want to update the Advanced Hunting query to include this group in the list of sensitive group list for this query and for the query from the previous blog.
If this was an unauthorized change, we should continue to investigate the user to see:
We can do this using data from all the products in Microsoft 365 Defender.
For more information on investigating threats see, https://docs.microsoft.com/en-us/microsoft-365/security/defender/incidents-overview?view=o365-worldw....
So to recap, you just created a query to show activities when a group is added to a sensitive group and then you created a custom detection policy. Congratulations! Feel free to save this query, then customize it further to suit your organization’s needs.
If you find the query and custom detection policy helpful, please leave a comment, or use the comment space to tell us what you’ve done to make this query even more powerful in your organization. Please let us know what areas you want to see us tackle next in Advanced Hunting. We can’t wait to hear your ideas. Happy hunting!
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