Is this flue dangerous?
I am guessing from your pictures that it is an air intake - I don't see any signs of air/moisture above the venting.  It is perfectly acceptable to pull air from your attic for a furnace.  In fact some might presuppose that by pulling the attic air you are increasing airflow in your attic.  As a home owner I would like to see a screen on it so that critters can't get in.

If this is an exhaust, then it needs to go out the roof.  I doubt it is an exhaust though since the exhaust looks to be right next to it and there are no signs of any airflow like I said before.  Your worry % should be very very low.

Also in my area we throw a couple of 90 degree elbows on this so it is pointing down.  That plus the screen makes it super hard to have any unwanted things fall into your furnace intake.