SS7 is a protocol that makes it possible for a person in one country to send text messages to someone in another country. Unfortunately the same functionality can be used to eavesdrop on conversations. In January, hackers used SS7 to redirect the text messages the banks used to send one-time passwords, using the passwords to transfer money out of the accounts.
"Criminals carried out an attack from a network of a foreign mobile network operator in the middle of January," a representative with Germany's O2 Telefonica told a Süddeutsche Zeitung reporter. "The attack redirected incoming SMS messages for selected German customers to the attackers." The unidentified foreign network provider has since been blocked, and affected customers were informed of the breach. The potential for widespread abuse of SS7 first came to light in 2008, but awareness remained largely limited. In 2014, The Washington Post explained how it was becoming increasingly easy for SS7 to be exploited by both government intelligence agencies and non-state actors. Last year, US TV news program 60 Minutes demonstrated the attack on Representative Lieu's number.