A shoplifting suspect was seriously injured yesterday when he leaped over a third-floor railing at a Cambridge mall and plummeted to the first floor after fleeing from security staff, police said. Cambridge police spokesman Dan Riviello said in e-mail that security guards stopped the man outside a clothing store at the CambridgeSide Galleria Mall about 3:40 p.m. The man, whose name is not being released because he has not been booked, fled and hopped over the railing, Riviello said. The suspect was apprehended by Cambridge police on the first floor. He suffered serious injuries to his legs, back, and neck, but was conscious and alert when taken into custody, Riviello said. The man was taken to a hospital and will be summoned to court at a later date on shoplifting and possibly other charges, Riviello said.