https://arstechnica.com/information-technology/2018/05/nefarious-actors-may-have-abused-routing-protocol-to-spy-on-us-phone-users/

The US Department of Homeland Security recently warned that malicious hackers may have targeted US phone users by exploiting a four-decades-old networking protocol used by cell phone providers around the world, according to a spokesman for US Senator Ron Wyden (D-Ore.). Meanwhile, the spokesman said, one of the nation’s major cellular carriers recently experienced a breach of that same protocol that exposed customer data.

Short for Signalling System No. 7, SS7 is the routing protocol that allows cell phone users to connect seamlessly from network to network as they travel throughout the world. With little built-in security and no way for carriers to verify one another, SS7 has always posed a potential hole that people with access could exploit to track the real-time location of individual users. In recent years, the threat has expanded almost exponentially, in part because the number of companies with access to SS7 has grown from a handful to thousands. Another key reason: hackers can now abuse the routing protocol not just to geolocate people but, in many cases, to intercept text messages and voice calls.