In a general aspect, a computer implemented method can include identifying, in a program code stored in computer memory, a location of a dynamic repair target code section for an assignment to a program variable, a value of which is known only at runtime. The method can also include modifying the program code at the identified location of the dynamic repair target code section to assign an immediate value different from a correct runtime value to the program variable. The method can further include inserting, into the program code in the computer memory, a dynamic repair probe code section that is executed before the dynamic repair target code and, at runtime, modifies the immediate value in the code to contain the correct runtime value when an attack has not occurred.