Following a low-key opening, McCormack gave the away side the lead in the 17th minute.
Charlton slowly worked their way back into the game and forged out a couple of half-chances before Simon Church came close to levelling the scores.
Rhoys Wiggins slipped the forward through with a good pass and he struck the outside of the post from a tight angle.
In the 42nd minute, Church was denied once more as his rasping effort was turned away by Paddy Kenny, but just seconds before the half-time whistle, Charlton were level.
Johnnie Jackson's free kick was headed away but it fell only as far as Stewart, who thumped a first-time volley into the back of the net.
Church almost turned the game entirely on its head in first-half stoppage time, but Tom Lees cleared the ball off the line.