Highly-rated right-back Long made his debut for the U's back in 2013 and is a graduate of the club's academy side.
He has made eighteen appearances for the club in that time, all in the Football League.
Beavers` manager Alan Dowson seemed pretty happy with the deal, telling the club`s official Pitchero website: "It's good to get someone in of his quality to add to the existing players we already have and we'll be glad of his experience in the next month or so."
Mischievously, Dowson had another reason for being happy with the loan deal. "Sam's a Sunderland supporter, like me and Graham the chairman, so he's going to fit it right away!"
Long will go into the squad for tomorrow's visit of Hungerford Town as Dowson will be looking for a good result to keep the pressure on Dartford at the top of the table.
In addition, Dowson has another signing lined up. "We've all the paperwork lined up for anther signing, but it won't go through yet as the player has got 'flu. We'll complete the deal next week when he's better".