Preventing Privilege Escalation

Abstract

Many operating system services require special privileges to execute their tasks. A programming error in a privileged service may open the door to system compromise in form of unauthorized acquisition of privileges. In the worst case, a remote attacker may obtain superuser privileges. In this paper, we discuss the methodology and design of privilege separation, a generic approach that lets parts of an application run without special privileges. Programming errors occurring in these now unprivileged parts of the application can no longer be abused to gain unauthorized privileges. Privilege separation is orthogonal to capability or role-based security systems and may be used to enhance the security of such systems even further. As a concrete example, the concept of privilege separation has been implemented in OpenSSH. We illustrate how separation of privileges reduces the amount of OpenSSH code that is executed with privileges. Privilege separation would have prevented past security vulnerabilities in OpenSSH including those that were unknown at the time of its implementation.