Cyber attacks on critical infrastructure have been an issue of importance in industry over the past several years, as well as a focus of academic research. Cyber attacks of various types and magnitude have been on the rise targeted specifically at power grids. A successful attack on the power grid could result in significant impacts including grid shutdown, cascading failures, damage to the infrastructure, and potential harm to people. Power grid infrastructure are critical in nature, they enable operations for residential, commercial, industrial, and government users across critical infrastructure such as water, communication, banking, transportation, manufacturing, and more. The compromise of these operations introduces threats, which span from economic to public safety. The Advanced Metering Infrastructure (AMI) is now being rapidly deployed throughout the power grid, and is an enabling technology for smart grid. Identifying the attack surface is a necessary step in achieving cyber security in smart grids and AMI. The purpose of this paper is to quantify and examine the cyber attack surface of AMI in order to lay the foundation for mitigating approaches to AMI cyber security.