An error was found in the X-Pack Security 5.3.0 to 5.5.2 privilege enforcement. If a user has either 'delete' or 'index' permissions on an index in a cluster, they may be able to issue both delete and index requests against that index.
https://discuss.elastic.co/t/x-pack-security-5-6-0-and-5-5-3-security-update/100089