B757 Captain reported an inflight upset and a minor injury to a Flight Attendant after encountering wake turbulence on descent into IAD in trail of a B767.

Narrative

On descent into IAD on the Hyper 9 Arrival we hit some wake turbulence. The First Officer (FO) was the flying pilot. The airplane rolled almost 40 degrees bank. He said 'upset;' clicked off the autopilot and put the aircraft back to the normal state. I asked ATC what we were following; and they advised us it was a heavy 767. We asked for more separation; and they gave us vectors to increase our distance from the heavy aircraft. After all this the Flight Attendant (FA) called us to let us know one of the flight attendants was injured. We asked if they were okay and if they needed any medical attention. The injured FA said she hurt her shoulder but did not need any immediate attention. We also asked if she needed to be met at the gate for medical aid. She said no and she just wanted to go to the clinic to have her shoulder checked out. After we parked the aircraft; I spoke with her; and she said her shoulder was sore and she just wanted to go to the clinic to have it checked out and have it documented. She walked off the jet with us with her bags in tow by herself without needing assistance to carry the bags.