On April 18th Amazon finally responded publicly with respect to the SIP attacks recently suffered from hosts within their EC2 service. Their response comes in the form of an informational security bulletin posted to their AWS Security Center.
There have been some recent discussions about SIP brute force attacks originating from Amazon EC2. We can confirm that several users reported SIP brute force attacks originating from a small number of Amazon EC2 instances about a week ago. It appears these attacks were designed to exploit security vulnerabilities in the SIP protocol. There is nothing specific about this attack that requires Amazon EC2. It was a brute force attack that could be launched from any computer on any network.