United Airlines flight 1400 encountered turbulence near Pharisburg, Ohio during descent into John Glenn Columbus International Airport (KCMH), Columbus, Ohio. One flight attendant was injured during the turbulence encounter. The airplane was not damaged. The captain was the pilot flying, and the first officer was the pilot monitoring. According to the flight crew, the flight experienced turbulence during the climb out, and areas of moderate to light chop during the first half of the flight, and it then encountered smooth air. The remainder of the cruise portion of the flight was uneventful. The flight crew began the final descent utilizing an RNAV arrival. According to the flight crew, weather conditions for the arrival were visual flight rules (VFR), no pilot reports (PIREPS) of turbulence were relayed to the flight crew by air traffic control (ATC), and no turbulence was forecasted for the arrival in the flight release weather information. The flight crew stated that during the approach, when the flight was at an altitude of about 12,000 ft-msl, they observed a scattered cloud layer with tops at around 10,000 ft-msl and could see the airport through the scattered layer. The flight crew stated they contacted the flight attendants early at around 11,000 ft-msl for final cabin checks and illuminated the seatbelt sign, since they anticipated the potential for some chop at the cloud tops during the descent. When the flight leveled at 10,000 ft-msl in the cloud tops, it encountered two strong jolts of moderate turbulence within seconds of each other. The autopilot system did not disconnect during the encounter. The flight crew stated the duration from the initial bump, followed by the second bump, then exiting the clouds entirely was approximately 8 seconds, and the flight crew requested a lower altitude to exit the turbulence. During this descent from 10,000 ft msl, the flight crew received an interphone call from the purser, stating that a flight attendant at the rear of the cabin was injured and that two medical personnel onboard were assisting. The first officer made a PIREP to ATC regarding the turbulence, then coordinated with the airport operations and control tower to ensure that an ambulance would meet the flight at their arrival gate. The flight landed uneventfully, and the flight attendant was later diagnosed with a fractured ankle.