They could use the vulnerability in the GSM network technology, which is used by billions of people in about 80 percent of the global mobile market, to make calls or send texts to expensive, premium phone and messaging services in scams, said Karsten Nohl, head of Germany's Security Research Labs.

Similar attacks against a small number of smartphones have been done before, but the new attack could expose any cellphone using GSM technology.

"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.

Attacks on corporate landline phone systems are fairly common, often involving bogus premium-service phone lines that hackers set up across Eastern Europe, Africa and Asia.

Fraudsters make calls to the numbers from hacked business phone systems or mobile phones, then collect their cash and move on before the activity is identified.

The phone users typically don't identify the problem until after they receive their bills and telecommunications carriers often end up footing at least some of the costs.

Even though Nohl will not present details of attack at the conference, he said hackers will usually replicate the code needed for attacks within a few weeks.

T-MOBILE, SFR LEAD RANKING

Mobile networks of Germany's T-Mobile (DTEGn.DE) and France's SFR (VIV.PA) offer their clients best protection against online criminals wanting to intercept their calls or track their movements, shows a new ranking Nohl will demonstrate at his presentation.

The new ranking, at gsmmap.org, lets consumers to see how their operators are performing and lets anyone to participate in measurement of their carriers' security.

Researchers reviewed 32 operators in 11 countries and rated their performance based on how easy it was for them to intercept the calls, impersonate someone's device or track the device.

"None of the networks protects users very well," Nohl said.

The sample is set to grow from 32 carriers dramatically next year as the tool enables anyone to participate in gathering of the data.

Nohl said mobile telecom operators could easily improve their clients' security, in many cases by just updating their software.

"Mobile network is by far the weakest part of the mobile ecosystem, even when compared to a lot attacked Android or iOS devices," he said.

Researchers reviewed operators in Austria, Belgium, the Czech Republic, France, Germany, Hungary, Italy, Morocco, Slovakia, Switzerland and Thailand.

(Additional reporting by Jim Finkle; Editing by Matt Driskill and Vinu Pilakkott)

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