Mark Coulburn, 30, of HMP Garth, Walton Lane, Leyland, did not attend his trial at Preston Crown Court.
He had denied wounding fellow inmate Billy Moriarty with intent to cause grievous bodily harm on July 28 last year, as well as the unauthorised possession of a weapon in prison.
Preston Crown Court was told both Mr Moriarty and Coulburn would not give an account to police after the incident.
Footage of the incident was caught on the prison’s CCTV system.
Moriarty was left with blood soaking through his shirt from several small puncture wounds to his upper left side and right shoulder blade.
A wound on his lower left back was bleeding profusely and an ambulance was called.
Prison officer Christopher Alexander heard the commotion from a nearby staff room and saw Coulburn pushing Moriarty, before “striking out” a number of times, with a jabbing motion.
He followed Coulburn back to his cell, where he was found standing near an open window.
Two officers later found a homemade sharpened metal blade with a blue tapped handle discarded on the concrete flooring outside.
It was dry, despite it raining that day.
A DNA profile matched Coulburn’s.
He will be sentenced at on October 9.