The difference between kernel mode and user mode gives a rudimentary form of security in the following manner. Convinced instructions could be executed only when the CPU is in kernel mode.
In the same way, hardware devices could be accessed only when the program is executing in kernel mode. Control over when interrupts could be Exercises enabled or disabled is also possible only when the CPU is in kernel mode.
As a result, the CPU has very limited capability when executing in User mode, thus enforcing protection of critical resources.