Giefer, 22, currently has a year left on his deal at the Esprit Arena, but he is reportedly being watched by several clubs, including the likes of Schalke 04 and Everton.
The Germany Under-21 international, who joined the club from Bayer Leverkusen last July, has produced a string of impressive displays for the Rhine outfit this season.
His agent Thomas Strunz told BILD newspaper: "Fabian is doing fine just here. He has a contract with the club beyond the end of the season.
"I am able to confirm that we have received several enquiries from clubs from in and outside Germany regarding Fabian's availability, but I'm not going to reveal any names."