In a computer system operable at more than one privilege level, an application is securely customized to use secret data without disclosing the secret data to a managing operating system. In operation, an integrity module executes at a higher privilege level than both the managing operating system and the application. After the managing operating system loads the application executable code, the integrity module injects the secret data directly into the instruction stream of the application executable code and then sets the memory location of the secret data as executable-only. As the application executes at the assigned privilege level, the instruction in the application directly accesses the secret data without performing any indirect memory access, thereby protecting the secret data from malicious attempts to read the secret data at a privilege level lower than the integrity module.