Only under two systems, both from base level 60. A player who has switched on PK Mode can attack you outside towns; they are marked the moment they strike, anyone may hit them back, and killing an innocent player turns them red and costs them a bounty. And during a Field Conquest war the drawn field is a battleground for an hour. Below level 60 nobody can attack you outside PvP rooms. The PK Mode article and the Events page have the rules.