Automatic Dependent Surveillance - Broadcast (ADS-B) is the next generation communication technology selected for allowing commercial and military aircraft to deliver flight information to both ground base stations and other airplanes. Today, it is already on-board of 80% of commercial aircraft, and it will become mandatory by the 2020 in the US and the EU. ADS-B has been designed without any security consideration --- messages are delivered wirelessly in clear text and they are not authenticated. In this paper we propose Securing Open Skies (SOS), a lightweight and standard-compliant framework for securing ADS-B technology wireless communications. SOS leverages the well-known \muTESLA protocol, and includes some modifications necessary to deal with the severe bandwidth constraints of the ADS-B communication technology. In addition, SOS is resilient against message injection attacks, by recurring to majority voting techniques applied on central community servers. Overall, SOS emerges as a lightweight security solution, with a limited bandwidth overhead, that does not require any modification to the hardware already deployed. Further, SOS is standard compliant and able to reject active adversaries aiming at disrupting the correct functioning of the communication system. Finally, comparisons against state-of-the-art solutions do show the superior quality and viability of our solution.