In a later interview with GQ the actor said he had a so-called butt double used for the revealing scene.
“The only time you saw my ass, it wasn’t my ass,” Harington said.
A crew member of similar height and hair color was hired to replace Harington when he broke his ankle and couldn’t shoot. The person who stepped in for Jon Snow has never been named and certainly did a darn good job as nobody saw through it.