Security paper by Marcin Olawski. Abstract: The GSM network is the biggest IT network on the Earth. Most of their users are connected to this network 24h a day but not many knows anything abut GSM security, how it works and how good it is. Most people blindly trust GSM security and send by the network not only theirs very private conversations and text messages but also their current location. This paper will describe how that information is guarded in 2G networks and how much of it an attacker can access without our permission or knowledge.