Default master user names for publicly accessible clusters can be a backdoor for hacking. While setting a custom master user name alone does not fully protect against attacks, restricting the root account only to privileged users and using additional password measures can add an additional layer of protection.
Follow the Getting information about cluster configuration docs to access your cluster configuration information. If the cluster has a master user name of awsuser, it is the default master user name. Modify the user name to a custom user name in the console.