Vehicle brake assist hold was engaged, foot still depressed on the pedal. The car released the brake hold and starting rolling at 5 mph even with my foot still on the brake. I had to let off the brake pedal and slam back on it and the car didn't stop in time and still hit the car in front of me. Very light impact but as a result led to the whole front of the car being out of shape (bumper, grille, headlights). Was aware of its deactivation but was not aware that the force of the brake pedal does not override the auto hold assist when the assist is deactivated.