One question which I heard often is: „From the security perspective what is to do when I use an ORACLE Exadata Database Machine?”
The answer is simple: You have to the same when you operate an ORACLE Database RAC System on an other Hardware witth Linux Operating system.
On the operating system side you have to secure your operating system. Change operating system default passwords. Create the correct access codes.
On the database side, use the database security. Change the database default passwords. Activate auditing. If you need more requirements you can buy the Advanced Security Option.
But all this is Exadata independent
The Storage Cell Server and the Infiniband Switch you also have to change the default passwords.
More about Database Security at the DOAG Security Day, at 22. September 2015 in Leipzig / Germany.