SDR-Based Network Impersonation Attack in GSM-Compatible Networks Despite the rapid change in cellular technologies, Mobile Network Operators (MNOs) keep a high percentage of their deployed infrastructure using Global System for Mobile communications (GSM) technologies. With about 3.5 billion subscribers, GSM remains as the de facto standard for cellular communications. However, the security criteria envisioned 30 years ago, when the standard was designed, are no longer sufficient to ensure the security and privacy of the users. Furthermore, even with the newest fourth generation (4G) cellular technologies starting to be deployed, these networks could never achieve strong security guarantees because the MNOs keep backwards- compatibility given the huge amount of GSM subscribers. In this paper, we present and describe the tools and necessary steps to perform an active attack against a GSM-compatible network, by exploiting the GSMprotocol lack of mutual authentication between the subscribers and the network. The attack consists of a so-called man-in-the- middle attack implementation. By using Software Defined Radio (SDR), open-source libraries and open- source hardware, we setup a fake GSM base station to impersonate the network and therefore eavesdrop any communications that are being routed through it and extract information from their victims. Finally, we point out some implications of the protocol vulnerabilities and how these can not be mitigated in the short term since 4G deployments will take long time to entirely replace the current GSM infrastructure.