Improved buffer overflow protection for a computer function call stack is provided by placing a predetermined ShadowKEY value on a function's call stack frame and copying the ShadowKEY, a caller EBP, and a return pointer are pushed onto a duplicate stack. The prologue of the function may be modified for this purpose. The function epilogue is modified to compare the current values of the ShadowKEY, caller EBP, and the return pointer on the function stack to the copies stored on the duplicate stack. If they are not identical, an overflow is detected. The preserved copies of these values may be copied back to the function stack frame thereby enabling execution of the process to continue. A function prologue and epilogue may be modified during compilation of the program.