What just happened? Street-level surveillance technologies such as automated license plate readers, facial recognition systems, and cell-site simulators (CSS) have become increasingly prevalent. Among these, CSS, also known as Stingrays or IMSI catchers, are particularly concerning due to their ability to masquerade as legitimate cellular towers, intercepting and logging sensitive information from mobile devices. As such, the Electronic Frontier Foundation (EFF) has developed an open-source tool called Rayhunter to help detect and understand the use of these devices. CSS operate by mimicking cell towers, tricking nearby phones into connecting to them instead of legitimate towers. This allows law enforcement to pinpoint phone locations with greater accuracy than other methods and log unique identifiers like IMSI numbers and IMEIs without involving phone companies. Some CSS can even intercept communications under certain conditions, including phone calls, text messages, and mobile internet traffic.However, the exact capabilities and deployment of CSS remain largely unknown due to a lack of transparency from manufacturers and law enforcement agencies, according to EFF.One of the most significant concerns with CSS is their potential to undermine privacy rights, particularly the Fourth Amendment in the U.S. These devices can be used to track individuals without their knowledge, often without a warrant, raising serious legal and ethical questions. In some cases, CSS have been used at protests and other gatherings, potentially infringing on First Amendment rights by surveilling large groups of people without probable cause.EFF has created Rayhunter to identify when these technologies are in use. Running on an Orbic mobile hotspot available for under $20, EFF has designed the tool to be user-friendly and accessible to individuals of all technical skill levels.Rayhunter intercepts and analyzes control traffic between the mobile hotspot and cell towers, looking for suspicious events such as forced downgrades to 2G networks, which are vulnerable to interception, or unusual IMSI requests that could indicate CSS activity. It alerts users to potential threats and allows them to review logs for further analysis. // Related StoriesRayhunter's interface is very simple, with a color-coded system indicating whether any suspicious activity has been detected. A green line (or blue for colorblind users) means no threats are detected, while a red signal indicates potential CSS activity.Users can access detailed logs via a web-based interface by connecting to the hotspot's Wi-Fi network or using a USB connection with Android Debug Bridge (ADB).Installing Rayhunter is relatively straightforward: download the software package, plug in the device, and run an installation script on Mac or Linux systems. The tool is open-source and available on GitHub under a GPL-3.0 license.By rolling out Rayhunter, EFF hopes to determine if CSS are used to surveil activities protected by the First Amendment, such as protests or religious gatherings, and to gather empirical data on the exploits used by these devices. This data will also help researchers understand how CSS exploit network vulnerabilities, enabling the development of better defenses against them.EFF also wants to see Rayhunter inform policy discussions and enhance legal protections against unauthorized surveillance, particularly in countries without robust free speech protections. In the U.S., for example, there have been attempts to introduce legislation like the Cell-Site Simulator Warrant Act, which would require warrants for CSS use, though such efforts have faced challenges. Recognition of the need for oversight is growing: The Department of Justice has implemented policies requiring search warrants for CSS use in many cases.