A German security expert says GSM phones have a vulnerability that hackers can exploit easily to take over the phone for calls or messaging, Reuters reported Tuesday.

The report came from Karsten Nohl, head of Germany’s Security Research Labs.

GSM is used by about four-fifths of mobile phones in operation worldwide, Reuters said.

“We can do it to hundreds of thousands of phones in a short timeframe,” Nohl told Reuters in advance of a presentation at a hacking convention in Berlin on Tuesday.

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