diesel fuel leaking from top front of tank

Did someone slice your filler hose to siphon the fuel? New vehicles have good blocks at the filler so it is harder just to stick a hose down the neck and siphon. Theives have gotten around this by cutting the filler hose and sticking their siphon hose in there. If that is not the case, then sounds like you have a loose hose or o-ring at the top of the tank.