Smartphones are susceptible to malware and carriers have enabled NSA snooping, but the prevailing wisdom has it there’s still one part of your mobile phone that remains safe and un-hackable: your SIM card.

Yet after three years of research, German cryptographer Karsten Nohl claims to have finally found encryption and software flaws that could affect millions of SIM cards, and open up another route on mobile phones for surveillance and fraud.

Nohl, who will be presenting his findings at the Black Hat security conference in Las Vegas on July 31, says his is the first hack of its kind in a decade, and comes after he and his team tested close to 1,000 SIM cards for vulnerabilities, exploited by simply sending a hidden SMS. The two-part flaw, based on an old security standard and badly configured code, could allow hackers to remotely infect a SIM with a virus that sends premium text messages (draining a mobile phone bill), surreptitiously re-direct and record calls, and — with the right combination of bugs — carry out payment system fraud.

Payment fraud could be a particular problem for mobile phone users in Africa, where SIM-card based payments are widespread. The deployment of so-called NFC payment technology, already slow to take off, could also be at risk, Nohl says, as well as the ability for carriers to track charges to each caller’s account.

There’s no obvious pattern to the flaw beyond the premise of an older encryption standard. “Different shipments of SIM cards either have [the bug] or not,” says Nohl, who is chief scientist at risk management firm Security Research Labs. “It’s very random.”

In his study, Nohl says just under a quarter of all the SIM cards he tested could be hacked, but given that encryption standards vary widely between countries, he estimates an eighth of the world’s SIM cards could be vulnerable, or about half a billion mobile devices.