Today's businesses are increasingly relying on the cloud as an alternative IT solution due to its flexibility and lower cost. Compared to traditional enterprise networks, a cloud infrastructure is typically much larger and more complex. Understanding the potential security threats in such infrastructures is naturally more challenging than in traditional networks. This is evidenced by the fact that there are limited efforts on threat modeling for cloud infrastructures. In this paper, we conduct comprehensive threat modeling exercises based on two representative cloud infrastructures using several popular threat modeling methods, including attack surface, attack trees, attack graphs, and security metrics based on attack trees and attack graphs, respectively. Those threat modeling efforts may provide cloud providers useful lessons toward better understanding and improving the security of their cloud infrastructures. In addition, we show how hardening solution can be applied based on the threat models and security metrics through extended exercises. Such results may not only benefit the cloud provider but also embed more confidence in cloud tenants by providing them a clearer picture of the potential threats and mitigation solutions.This section briefly reviews several popular threat models and existing security metrics that will be applied in this paper, including attack surface, attack tree, attack graph, attack tree-based metric (ATM), and Bayesian network (BN)-based metric.-Attack surface: Originally proposed as a metric for software security, an attack surface captures software components that may lead to potential vulnerabilities, including entry and exit points (i.e., methods in a software program that either take user inputs or generate outputs), communication channels (e.g., TCP or UDP), and untrusted data items (e.g., configuration files or registry keys read by the software) [7]. Since the attack surface requires examining the source code of a software, due to the complexity of such a task, most existing work applies the concept in a high-level and intuitive manner. For example, six attack surfaces are said to exist between an end user, the cloud provider, and cloud services [8], although the exact meaning of such attack surface is not specified.-Attack tree: While the attack surface focuses on what may provide attackers initial privileges or accesses to a system, attack trees demonstrate the possible attack paths which may be followed by the attacker to further infiltrate the system [9].