29 CFR § 1606.3 - The national security exception.
It is not an unlawful employment practice to deny employment opportunities to any individual who does not fulfill the national security requirements stated in section 703(g) of title VII. 1
1 See also, 5 U.S.C. 7532, for the authority of the head of a Federal agency or department to suspend or remove an employee on grounds of national security.