In 2014, researchers demonstrated that SS7, which was created in the 1980s by telcos to allow cellular and some landline networks to interconnect and exchange data, is fundamentally flawed. Someone with internal access to a telco – such as a hacker or a corrupt employee – can get access to any other carrier’s backend in the world, via SS7, to track a phone’s location, read or redirect messages, and even listen to calls.
In this case, the attackers exploited a two-factor authentication system of transaction authentication numbers used by German banks. Online banking customers need to get a code sent to their phone before funds are transferred between accounts.
The hackers first spammed out malware to victims’ computers, which collected the bank account balance, login details and passwords for their accounts, along with their mobile number. Then they purchased access to a rogue telecommunications provider and set up a redirect for the victim’s mobile phone number to a handset controlled by the attackers.
Next, usually in the middle of the night when the mark was asleep, the attackers logged into their online bank accounts and transferred money out. When the transaction numbers were sent they were routed to the criminals, who then finalized the transaction.