A system and method is provided for reducing the complexity and improving the performance of enforcing security restrictions on the execution of program code in a runtime environment. In a preferred embodiment, units of executable code, such as methods or functions, are classified by "security level." Code units belonging to a "trusted" security level may call any other code unit in the runtime environment, but other security levels are restricted in the code units they can call. In a preferred embodiment, the security levels are represented by corresponding permission objects. Each permission object that is associated with a particular security level includes a numerical value that denotes that security level. Security policies can be enforced with respect to caller and callee code units by comparing numerical values of corresponding permission objects. This security level scheme also improves runtime performance by making it unnecessary to check individually-defined permissions in many cases.