Webo, who came on as a second-half replacement for Jean Makoun, provided Samuel Eto'o and Landry Nguemo with perfect back passes as Cameroon twice fought back to finish second in Group D.
They will take on Egypt, who beat them in the 2008 final, in Benguela, while Zambia, who went through at the top of Group D on goals scored after beating Gabon 2-1, will face Nigeria in Lubango.
Tunisia, eliminated after drawing their three games, had gone ahead in the first minute through Amine Chermiti and again in the 63rd when defender Aurelien Chedjou netted into his own goal.
Gabon, Cameroon and Zambia all finished on four points, with Zambia winning a mini-league between them on goals scored (4), ahead of Cameroon (3) and Gabon (2).
Veteran Rigobert Song, whose blunders cost Cameroon two goals in their first two games, started on the bench but that did not prevent the Indomitable Lions from conceding the early goal.
The central defence was caught snoozing when striker Chermiti connected with a Khaled Souissi cross to head home after 50 seconds.
Cameroon piled on the pressure but proved too clumsy as Tunisia's back four held on.
Tunisia keeper Aymen Mathlouthi was lucky to escape a straight red card after denying Eto'o a clear chance when he handled the ball just outside the box on the half hour.
Although Cameroon enjoyed possession, Tunisia had another chance shortly before halftime when Yassine Mikari collected a through pass from Issam Jomaa but his shot went just wide.
Webo was brought on just after the break to replace midfielder Jean Makoun and the move immediately paid off as the Mallorca striker's back pass found Eto'o, who coolly slotted home to level the contest in the 47th.
Centre back Chedjou, however, lobbed his own keeper in an attempt to clear a Mikari cross as Tunisia regained the lead in the 63rd minute.
One minute later, Nguemo equalised for Cameroon when he fired past Mathlouthi from another Webo back pass.
Tunisia finished with 10 men after defender Ammar Jemel was sent off for a second bookable offence.
