Bitcoin isn’t nearly as safe as most users believe, being vulnerable to an attack that could allow a small group of “selfish Bitcoin miners” to effectively wrest control of the virtual currency – all it would take is a group of determined cheaters.

This vulnerability theory is the discussion of a new research paper published by Cornell University post-doctoral fellow Ittay Eyal and Professor Emin Gun Sirer.

The problem is that Bitcoin has a major design flaw due to the way individual bitcoins are created – these are ‘mined’ by groups (known as Bitcoin miners) that use multiple computers to tackle complex math puzzles. The system is organized in such a way that one of these groups will solve the puzzle every ten minutes, with the winner receiving 25 bitcoins for their efforts. Because these groups are effectively in competition with one another, the virtual currency is fairly evenly distributed.

But according to Eyall and Sirer, there’s a fundamental weakness in this system that could easily be exploited, giving one group a greater chance of solving these puzzles than rival Bitcoin miners. Once a puzzle is solved, this is then supposed to be stored in a public log known as a “blockchain,” which effectively notifies everyone to move onto the next puzzle. But the solutions don’t actually have to be publicized – a group of ‘selfish’ miners could keep it a secret, move onto the next puzzle, while everyone else is left behind mining in the wrong place.

The authors warn that this vulnerability could easily be exploited by a group of powerful miners with large computing resources to solve the puzzles. The idea is that the bigger the group, the more resources they have, the more puzzles they can solve – if such a group were to become ‘selfish’ and not tell anyone else when it had solved previous puzzles, this ‘headstart’, combined with their computing power, could allow it to take control of the currency.

“Once the system veers away from the happy mode where everyone is honest, there is no force that opposes the growth of really large pools that command control of the currency,” wrote Prof Surer

Should one group take over the creation of Bitcoin in this way, the net effect is that the virtual currency would become much the same as the dollar, the pound or the euro – a currency that’s controlled by a single, powerful central bank. Such a group could then manipulate the value of Bitcoin, simply by flooding the system with bitcoins or withholding them. If that were to happen, it would undermine one of the main reasons that people use the cryptocurrency – because its free from centralized authority or government control.

“No one wants to bring down Bitcoin,” writes Eyal. “But if you know you can increase your revenue by a bit, you’re going to join the selfish pool.”

Not everyone agrees with the theory though. Vitalik Buterin, technical editor of Bitcoin Magazine, told the BBC that he is skeptical that such an attack would work in practice, because it would require that attackers change fundamental BitCoin software to manipulate the information sharing with the network.

Fortunately, even if Buterin is wrong, there is an easy solution – the Bitcoin community could just change the rules, so that the total mining power of one group cannot exceed one-quarter of the entire mining community’s power. This, together with a simple update to Bitcoin’s software, would be enough to prevent any group from assuming control of the cryptocurrency.