The visitors pressured their in-form hosts and were rewarded when they took the lead with a fortuitous goal.
Stanley defender Tom Davies' clearance hit Jimmy Spencer 12 yards out and the ball bounced into the bottom corner.
The home side grabbed a vital equaliser when on-loan winger Tariqe Fosu found the bottom corner from 10 yards after a quick breakaway.
Accrington slipped from third to fifth in the table, with Cambridge still 12th, five points off the play-off places.
Accrington lost vital ground in the League Two automatic promotion race as they were held at home by Cambridge.