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Introduction
Imagine an AWS user in your environment escalates privileges by assuming a role (calling sts:AssumeRole) and performs a malicious action. How will you know in the first place and how will you find the offending user in order to remediate the situation?
CloudTrail of course. But you find that the event logged for the malicious action tells you the role and not necessarily the original user. To find that user, you need to go back to CloudTrail and find a different, earlier event logged for the AssumeRole call. Finding and correlating two events in CloudTrail or your SIEM is not necessarily difficult, as long as you have CloudTrail configured properly, secured it, have its events in an optimized and indexed data store with enough retention, and know what to look for.
Well, AWS has made all of our lives a little easier. On April 21, 2020, AWS released a new IAM policy condition that, when configured properly, will force the user assuming the role to include their username, which will then be logged in the event every time the user performs an action with that role. Now, only one event needs to be looked at and the user is clearly logged in one of the CloudTrail event fields.
This blog post clarifies how to make use of this new feature to improve clarity of CloudTrail logging for the purposes of incident response and investigations. This will also make it easier to do any analysis on CloudTrail events since identifying the username/owner of a role-based action no longer requires correlation with another event.
The Old Way
CloudTrail Action Event
Here’s a CloudTrail event for a user that has escalated privileges with sts:AssumeRole and is listing objects in an S3 bucket:
We know the action taken is ListObjects on the bucket, MySensitiveBucket, but we don’t know who exactly is taking this action. We know they are using a role, MyBucketRole, but that’s about it.
CloudTrail AssumeRole Event
To figure out “who” or which IAM user is behind this, we need to go back to CloudTrail and find the earlier event logged when the user actually called sts:AssumeRole. This could be up to 36 hours before the original action event. Assuming we have CloudTrail properly configured and all events stored in an indexed/optimized datastore, perhaps a SIEM, we can query on the same accessKeyId and eventName = “AssumeRole” to find an event like this: