The 43-year-old believes his former club can target the Blaugrana's defensive frailties on Tuesday night.
The Blaugrana only just survived the quarterfinals of the competition as they beat Paris Saint-Germain on the away-goals rule and Kahn believes Bayern can take advantage of Barca's struggles at the back in order to make it to the club's third Champions League final in four years.
However, he stressed that Barcelona cannot be taken lightly as the club's star players always deliver when it matters most.
"Barcelona have shown this season how vulnerable they can be at the back. It's the moment of truth for Bayern," Kahn writes in his Bild column. "However, in the crucial moments, [Lionel] Messi, [Andres] Iniesta and Xavi have always pulled out world-class performances. In my opinion, the tie will be a very open one.
"Exiting the competition would be a huge disappointment to the Bayern fans and the players after such a dominant season."