Automatic Dependent Surveillance – Broadcast (ADS-B) system is based on wireless communications protocol to broadcast aircraft identification and navigational information via data link. Effectively mandated by almost all regional civil aviation regulatory bodies worldwide, ADS-B is essentially defining the future of many international Air Traffic Management programs like Global Plan Initiatives, NextGen ATS and Single European Sky ATM Research. Aircraft determine position from navigation satellites, merge these with identity and classification information for further broadcast on open / un-encrypted format over standard surveillance channel using ADS-B equipped transponders. Information collected by multiple ground, air and space-based ADS-B receivers is then fused to form complete surveillance picture for aircraft tracking to be used by Ground Controllers and Pilots, enabling effective Air Traffic Management. Additionally, this surveillance data is also available over internet for public viewing on PCs, laptops or even smartphones. Due to its open architecture and broadcast nature, ADS-B data transmitted over the air remains vulnerable and prone to unwarranted security and privacy issues. With commercial availability of inexpensive Software Defined Radios (SDRs), stated vulnerabilities can be further exploited through any or all attacks applicable to wireless communications medium, subsequently jeopardizing Air Traffic Services and risking flight safety of passengers. International Civil Aviation Organization, central to air transport policy and standardization worldwide, has also acceded to the security issues related to availability, confidentiality and integrity of ADS-B in their documentations. It has recommended to undertake security risk assessment studies in coordination with appropriate state organization to devise ways and means to mitigate the attack effectiveness. In cognizance to same, this survey article is an effort to compile information contained in various research proposals and industrial documents. It is aimed to understand ADS-B systems and its vulnerabilities, identifying threat levels and attacks, recent developments and analysis of developed or scholarly proposed countermeasures to secure ADS-B transmitted data, with or without altering existing protocol or infrastructure.