Catalyzes the hydrolysis of 3-deoxy-D-manno-octulosonate 8-phosphate (KDO 8-P) to 3-deoxy-D-manno-octulosonate (KDO) and inorganic phosphate in vitro. Also catalyzes the dephosphorylation of phospho-tyrosine in vitro. 3-deoxy-alpha-D-manno-2-octulosonate-8-phosphate + H2O = 3-deoxy-alpha-D-manno-oct-2-ulosonate + phosphate Carbohydrate biosynthesis; 3-deoxy-D-manno-octulosonate biosynthesis; 3-deoxy-D-manno-octulosonate from D-ribulose 5-phosphate: step 3/3. Bacterial outer membrane biogenesis; lipopolysaccharide biosynthesis. Homotetramer. Belongs to the KdsC family.