https://www.federalregister.gov/documents/2020/12/15/2020-27739/providing-an-order-of-succession-within-the-department-of-defense

Order of Succession. (a) Subject to the provisions of section 2 of this order, the following officials of the Department of Defense, in the order listed, shall act as and perform the functions and duties of the office of the Secretary of Defense (Secretary) during any period in which the Secretary has died, resigned, or otherwise become unable to perform the functions and duties of the office of the Secretary, ………..