Leaking around any vent is usually due to frost formation and thawing. Without proper insulation the venting system will frost up when the warm moist air enters the cold attic space. On a warm day this frost will melt inside the vent line and run back down into the fan or the ceiling. A lot of builders place a water trap in the vent line to catch this water but this is not a good idea in my opinion. If the water lies in the pipe in the cold attic it will soon form a block of ice. Once this ice fills the trap the fan no longer moves any air. This is especially bad in a bathroom because the fan can no longer move out the odors or the excessive moisture. The best practice is to ensure the line and vent is properly insulated so the heat stays in the pipe until it exits the roof.