What was found
(CAN) WHILE TAXING IN PREPARATION FOR TAKEOFF, CREW NOTED SMOKE AROUND AC, TOWER REPORTED SMOKE, FLAMES COMING FROM AFT AREA OF AC UNDER NR 2 ENG. APU WAS SHUTDOWN, CREW THEN ELECTED TO TAXI OFF ACTIVE TAXIWAY TO A NON ACTIVE TAXIWAY. SHORTLY AFTER STOPPING AC, AN APU FIRE MESSAGE WAS POSTED ON EICAS. CREW COMPLETED QRH PROCEDURE FOR APU FIRE. CREW DISCHARGED AT LEAST ONE FIRE BOTTLE, AND FIRE INDICATIONS EXTINGUISHED. CFR RESPONDED AND DISCHARGED HALON INTO APU ENCLOSURE. CREW THEN EVACUATED PASSENGERS THROUGH MAIN CABIN DOOR EXIT USING QRH PROCEDURE. ONE PASSENGER SUSTAINED A CUT FINGER. NO OTHER INJURIES REPORTED. THE APU WAS REPLACED, AC RETURNED TO SERVICE.