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Bitcoin Gold (BTG) appears to be the target of a 51 percent attack, leading to the theft of approximately $17.5 million from cryptocurrency exchanges.

Traders of the coin, established in 2017 through a hard fork of the main Bitcoin (BTC) network, have been tracing the attack since last week.

Scrutiny of unusual activity on the network has revealed the scheme, in which “double spending” attacks have been launched against cryptocurrency exchanges trading the virtual currency.

According to the Bitcoin Gold team, exchanges rather than individual users have been targeted.

The first attack was recorded on 16 May and have now appeared to have ceased.

“An unknown party with access to very large amounts of hashpower is trying to use “51 percent attacks” to perform “double spend” attacks to steal money from exchanges,” the team says. “We have been advising all exchanges to increase confirmations and carefully review large deposits. There is no risk to typical users or to existing funds being held.”

51 percent attacks force reorganization in the blockchain. While not the result of a security flaw or vulnerabilities, the attack can — theoretically — be launched against any blockchain.

If a threat actor manages to wrestle control of over 50 percent of a network’s computing power, they are able to both modify and exclude transactions of their own coins from blocks.

Attackers are then able to double spend, in which transactions are…