This paper describes a secure execution scheme for mobile programs running in networked appliances. The potential threat we are considering is the malicious behaviour of the mobile programs. Our security architecture, called Taurus-1, adopts monitored execution as a basic mechanism for enforcing security policies at runtime. To describe a policy for this architecture, we designed an algebraic policy description language Polaris, in which we can specify a policy as a process consists of abstract events in a monitored program. The program is modified beforehand for effective detection of its malicious behaviour. We discuss how Taurus-1 can provide a security enhancement mechanism for context-aware systems such as networked appliances, and discuss how to specify security policies for them.