Systems and methods, including computer software adapted to perform certain operations, can be implemented for preventing content received from non-trusted sources from accessing protected data. A sequence of instructions and multiple permission indicators associated with the sequence of instructions are received. One or more of permission indicators are associated with a protected activity. An instruction within the sequence of instructions associated with the protected activity is identified. A determination is made whether execution of the identified instruction is permitted based, at least in part, on the one or more permission indicators, and the protected activity is performed if execution of the identified instruction is permitted.