A system for providing security mechanisms for secure execution of program code is described. The system may be configured to maintain a plurality of virtual machine instances. The system may be further configured to receive a request to execute a program code and allocate computing resources for executing the program code on one of the virtual machine instances. One mechanism involves executing program code according to a user-specified security policy. Another mechanism involves executing program code that may be configured to communicate or interface with an auxiliary service. Another mechanism involves splitting and executing program code in a plurality of portions, where some portions of the program code are executed in association with a first level of trust and some portions of the program code are executed with different levels of trust.