The 28-year-old, who made 25 appearances for Spurs between 2013 and 2015, is out of contract at German side Hamburg at the end of the month and recently confirmed his intention to move on.
"After five intensive years at HSV, the signs are currently saying goodbye," he said. "I'm at the age where I am playing my best football and would like to be part of a great project again. We are currently in the active talks with relevant options."
According to Bild, Cardiff - relegated from the Premier League at the end of last season - are in pole position for his signature but face "intense" competition from clubs on the continent including PAOK, Fenerbahce and Galatasaray, as well as MLS side Chicago Fire.
The Germany international is thought to be open to the idea of playing in the second tier, although he is only willing to move to a club that can seriously compete for promotion to the Premier League.