Signaling System 7 (SS7) is vulnerable to design flaws that could allow hackers to listen in private phone calls and read text messages on a potentially vast scale, despite the most advanced encryption used by cellular networks. The design flaws in SS7 have been in circulation since 2014 when a team of researchers at German Security Research Labs alerted the world to it. O2 Telefonica has confirmed that the same SS7 weaknesses have recently been exploited by cybercriminals to bypass two-factor authentication (2FA) banks used to prevent unauthorized withdrawals from users bank accounts.
Source: https://thehackernews.com/2017/05/ss7-vulnerability-bank-hacking.html