The latest revelation was made in a new documentary film based on the memoirs of the Pope's personal secretary, Stanislaw Dziwisz.
He said that during the Pope's visit to a shrine in Fatima in Portugal, the priest came at the pontiff with a bayonet.
According to Cardinal Dziwisz, the priest managed to draw blood with the weapon but was overcome by the Pope's security guards.
"I can now reveal that the Holy Father was wounded. When we got back to the room [in the Fatima sanctuary complex] there was blood," he says in the documentary.
Krohn was sentenced to six and a half years for the attack
The Pope did not reveal the injury at the time and continued with the trip.
There has been no comment from the Vatican on the cardinal's revelations.
The documentary film, Testimony, pulls together material from the Pope's private life as related by Cardinal Dziwisz.
He is now the cardinal of Krakow in Poland and served as John Paul's private secretary for nearly 40 years, including all his 27 years as Pope.
The 90-minute film marks the first time the Vatican has opened its doors to foreign cameras.
Pope Benedict XVI will watch a preview later on Thursday.
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