Authority Analysis for Least Privilege Environments
Toby Murray and Gavin Lowe
The rise of limited-privilege environments has been accompanied by the emergence of vulnerabilities in which a subject is able to maliciously wield their limited privileges to indirectly cause unwanted effects. Unfortunately, conventional safety analyses for access control systems are ill-equipped to deal with this problem because they do not detect the indirect effects that a subject can cause, but merely the permissions a subject can acquire. We present a technique that characterises a subject's authority as all of the effects they can cause to occur. Our technique is based on an analysis of causation, applied to a CSP model of a system. These analyses can be expressed as CSP refinements and, hence, automatically performed by a refinement-checker such as FDR. We demonstrate the ability of our technique to successfully identify excess authority by examining the 'Confused Deputy' scenario, whose vulnerability goes undetected with conventional safety analyses.