In freezing temperatures at the Riverside, Boro failed to find the firepower to turn possession into enough goals to close the gap to the Sky Bet Championship play-off spots.
And it looked as if Rotherham, still rooted to the foot of the table, had made Michael Carrick's men pay with Cafu's 59th-minute opener.
The 30-year-old Portuguese winger's first goal of the season looked like it could end a 14-match winless run away from home dating back to November 2022.
But Forss' powerful strike in the 82nd minute levelled things up to secure a point for the hosts.
Middlesbrough controlled things by and large from start to finish but reached half-time goalless, yet Rotherham could easily have taken an early lead.
Jordan Hugill, the former Middlesbrough striker and Teessider, wasted a fantastic chance from just outside the box. He spotted goalkeeper Tom Glover off his line but his lob from 22 yards out dropped wide.
After that the home side, who lost winger Isaiah Jones through injury early on after he fell under a Hugill shove, created plenty of openings but lacked the scoring touch.
Defender Matt Clarke headed wide from a corner while Finn Azaz missed the target after he had cleverly been played in behind the Rotherham backline.
Sam Greenwood forced goalkeeper Viktor Johansson into a flying save to his right from distance and that was moments before the forward appeared to be pulled back in the area by defender Peter Kioso.
Despite Boro's complaints, referee Darren Bond waved play on and dismissed any claims for a penalty.
Before the break substitute Forss was denied by Johansson following an incisive Hayden Hackney pass but otherwise Rotherham were pretty solid.
Millers head coach Leam Richardson's tactics continued to frustrate Middlesbrough after the restart and after a few more Middlesbrough runs came to nothing, Rotherham broke the deadlock just before the hour.
Hugill's excellent pass with the outside of his boot after strong play from Jamie Lindsay was perfectly weighted for Cafu to run on to and finish first time low and beyond Glover.
After that Rotherham attempted to slow things down again and Johnasson was booked for taking too long on a goal-kick after full-back Luke Ayling had fired wide from the edge of the area.
Matt Crooks, Josh Coburn and Lewis O'Brien were introduced in a bid to turn things around and within seconds of the latter pair's arrival on the pitch, Forss levelled.
Morgan Rogers, who has reportedly been the subject of Aston Villa bids this week, burst into life. His flick was perfect for Forss to hammer inside Johansson's near post with eight minutes left.
Middlesbrough thought they had won it in stoppage time when Coburn slid in to finish, but the flag went up for offside because Crooks had attempted to make contact with the cross at the near post.