The usage of public Wi-Fi hotspots has become a common routine in our everyday life. They are ubiquitous and offer fast and budget-friendly connectivity for various client devices. However, they are exposed to a severe security threat: since 802.11 identifiers (SSID, BSSID) can be easily faked, an attacker can setup an evil twin, i.e., an access point (AP) that users are unable to distinguish from a legitimate one. Once a user connects to the evil twin, he inadvertently creates a playground for various attacks such as collection of sensitive data (e.g., credit card information, passwords) or man-in-the-middle attacks even on encrypted traffic. It is particularly alarming that this security flaw has led to the development of several tools that are freely available, easy to use and allow mounting the attack from commodity client devices such as laptops, smartphones or tablets without attracting attention.
In this paper we provide a detailed overview of tools that have been developed (or can be misused) to set up evil twin APs. We inspect them thoroughly in order to identify characteristics that allow them to be distinguished from legitimate hardware-based access points. Our analysis has discovered three methods for detecting software-based APs. These exploit accuracy flaws due to emulation of hardware behavior or peculiarities of the client Wi-Fi hardware they operate on. Our evaluation with 60 hardware APs and a variety of tools on different platforms reveals enormous potential for reliable detection. Furthermore, our methods can be performed on typical client hardware within a short period of time without even connecting to a potentially untrustworthy access point.