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Policy Enforcement via Program Monitoring (thesis)

Report ID:
April 2006
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One way to guarantee that software behaves securely is to monitor programs at run time and check that they dynamically adhere to constraints specified by a security policy. Whenever a program monitor detects that untrusted software is attempting to execute a dangerous action, it takes remedial steps to ensure that only safe code actually gets executed. This thesis considers the space of policies enforceable by monitoring the run-time behaviors of programs and develops a practical language for specifying monitors' policies.

In order to delineate the space of policies that monitors can enforce, we first have to define exactly what it means for a monitor to enforce a policy. We therefore begin by building a formal framework for analyzing policy enforcement; we precisely define policies, monitors, and enforcement. Having this framework allows us to consider the enforcement powers of program monitors and prove that they enforce an interesting set of policies that we define and call the infinite renewal properties. We show how, when given any reasonable infinite renewal property, to construct a program monitor that provably enforces that policy.

In practice, the security policies enforced by program monitors grow more complex both as the monitored software is given new capabilities and as policies are refined in response to attacks and user feedback. We propose dealing with policy complexity by organizing policies in such a way as to make them composeable, so that complex policies can be specified more simply as compositions of smaller subpolicy modules. We present a fully implemented language and system called Polymer that allows security engineers to specify and enforce composeable policies on Java applications. We also formalize the central workings of Polymer by defining an unambiguous semantics for our language.

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