Biblio

The principle of least authority states that each component of the system should be given authority to access only the information and resources that it needs for its operation. This principle is fundamental to the secure design of software systems, as it helps to limit an application’s attack surface and to isolate vulnerabilities and faults. Unfortunately, current programming languages do not provide adequate help in controlling the authority of application modules, an issue that is particularly acute in the case of untrusted third-party extensions. In this paper, we present a language design that facilitates controlling the authority granted to each application module. The key technical novelty of our approach is that modules are firstclass, statically typed capabilities. First-class modules are essentially objects, and so we formalize our module system by translation into an object calculus and prove that the core calculus is typesafe and authority-safe. Unlike prior formalizations, our work defines authority non-transitively, allowing engineers to reason about software designs that use wrappers to provide an attenuated version of a more powerful capability. Our approach allows developers to determine a module’s authority by examining the capabilities passed as module arguments when the module is created, or delegated to the module later during execution. The type system facilitates this by identifying which objects provide capabilities to sensitive resources, and by enabling security architects to examine the capabilities passed into and out of a module based only on the module’s interface, without needing to examine the module’s implementation code. An implementation of the module system and illustrative examples in the Wyvern programming language suggest that our approach can be a practical way to control module authority.

In today's systems, restricting the authority of untrusted code is difficult because, by default, code has the same authority as the user running it. Object capabilities are a promising way to implement the principle of least authority, but being too low-level and fine-grained, take away many conveniences provided by module systems. We present a module system design that is capability-safe, yet preserves most of the convenience of conventional module systems. We demonstrate how to ensure key security and privacy properties of a program as a mode of use of our module system. Our authority safety result formally captures the role of mutable state in capability-based systems and uses a novel non-transitive notion of authority, which allows us to reason about authority restriction: the encapsulation of a stronger capability inside a weaker one.