How talkative is your mobile device?

The IEEE 802.11 standard defines Wi-Fi probe requests as a active mechanism with which mobile devices can request information from access points and accelerate the Wi-Fi connection process. Researchers in previous work have identified privacy hazards associated with Wi-Fi probe requests, such as leaking past access points identifiers and user mobility. Besides several efforts to develop privacy-preserving alternatives, modern mobile devices continue to use Wi-Fi probe requests. In this work, we quantify Wi-Fi probe requests' threat to privacy by conducting an experimental study of the most popular smartphones in different settings. Our objective is to identify how different factors influence the probing frequency and the average number of broadcasted probes. Our conclusions are worrisome: On average, some mobile devices send probe requests as often as 55 times per hour, thus revealing their unique MAC address at high frequency. Even if a mobile device is not charging and in sleep mode, it might broadcast about 2000 probes per hour. We also evaluate a commercially deployed MAC address randomization mechanism, and demonstrate a simple method to re-identify anonymized probes.