McCormack struck with a left-foot shot with just eight minutes left - Rangers' only attempt on target - to salvage the unlikeliest of points.
It was scarcely deserved on the balance of play but it gives Rangers genuine hope of reaching the knock-out stages.
Porto dominated from the outset and came close after only seven minutes when Lisandro beat Marvin Andrews to a diving header but the ball drifted over the top with Ronald Waterreus beaten.
But for all their possession, the Portuguese side did not create too much in the way of clear-cut chances in the first half.
Rangers sat deep and did not offer much in terms of attack and indeed it was not until five minutes before the break that they managed to get the ball into the Porto penalty area.
Diego saw a shot blocked by Andrews and Waterreus was grateful to see the ball spin away for a corner kick.
Ricardo Quaresma was giving Fernando Ricksen a torrid time with his footwork on the left and the Rangers player was forced to give away a string of free-kicks and was fortunate to escape a booking.
Another dangerous cross from Quaresma after 32 minutes flicked dangerously off the head of Andrews but Lisandro could not get a touch at the back post.
Quaresma wasted a great chance to open the scoring six minutes later when he cut inside Ricksen but selfishly shot wildly into the side-netting when Lisandro was unmarked in the centre.
But Rangers held out until half-time to give them hope they could still take something from the tie.
After 59 minutes, their task became more daunting as Porto moved in front. Jose Bosingwa provided the cross for Lisandro to score with a diving header at the back post.
Rangers finally had a shot at goal after 69 minutes when the ball broke to Barry Ferguson outside the area but he blasted his effort high and wide.
Then came McCormack's dramatic intervention when he swept the ball into the net from eight yards after Chris Burke had headed the ball into his path.