In late 2014, hackers stole information linked to 500 million Yahoo users but the cyber crime was not revealed till recently. A hacker offered information for sale on the Darknet at the end of August. The accounts of social networks such as LinkedIn and tumblr are thought to have been hacked into by the same group. Many users do not know whether they were affected, and what the consequences may be. We find out from DW's IT security expert Ulf Bürger how users can better protect their data.