Page "Cahn–Ingold–Prelog priority rules" Paragraph 10

2.

If there is a tie, we must consider the atoms at distance 2 from the stereocenter — as a list is made for each group of the atoms bonded to the one directly attached to the stereocenter.

Each list is arranged in order of decreasing atomic number.

Then the lists are compared atom by atom ; at the earliest difference, the group containing the atom of higher atomic number receives higher priority.