Least-Privilege Calls to Amazon Web Services

We address least-privilege in a particular context of public cloud computing: calls to Amazon Web Services (AWS) Application Programming Interfaces (APIs). AWS is, by far, the largest cloud provider, and therefore an important context in which to consider the fundamental security design principle of least-privilege, which states that a thread of execution should possess only those privileges it needs. There have been reports of over-privilege being a root cause of attacks against AWS cloud applications, and a least-privilege set for an API call is a necessary building-block in devising a least-privilege policy for a cloud application. We observe that accurate information on a least-privilege set for an invoker of a method to possess is simply not available for most such methods in AWS. We provide a meaningful characterization of least-privilege in this context. We then propose techniques to determine such sets, and discuss a black-box process we have devised and carried out to identify such sets for all 707 API methods we are able to invoke across five AWS services. We discuss a number of interesting discoveries we have made, some of which are surprising and some alarming, that we have reported to AWS. Our work has resulted in a database of least-privilege sets for API calls to AWS, which we make available publicly. Developers can consult our database when configuring security policies for their cloud applications, and we welcome contributors that augment our database. Also, we discuss example uses of our database via an assessment of two repositories and two full-fledged serverless applications that are available publicly and have policies published alongside. We observe that the vast majority of policies are over-privileged. Our work contributes constructively to securing cloud applications in the largest cloud provider.