AWS CloudTrail Now Tracks Cross-Account Activity to Its Origin

You can use AWS Identity and Access Management (IAM) roles and AWS Security Token Service (STS) to set up cross-account access between AWS accounts. When you assume an IAM role in another AWS account to obtain cross-account access to services and resources in that account, AWS CloudTrail logs the cross-account activity. Starting today, CloudTrail logs AssumeRole calls in the role-owning account (the account being accessed), including the unique ID of the IAM entity (a user or role) assuming the role in the account being accessed. This additional information helps you identify the entity that requested cross-account access and then trace its subsequent cross-account activity.
Quelle: aws.amazon.com