Singapore-based cryptocurrency exchange Coinhako has restricted user withdrawals following a reported “sophisticated attack.”

Coinhako, which is backed by billionaire investor and bitcoin bull Tim Draper, told users on Friday that account withdrawals had temporarily been disabled.

A spokesperson initially said the suspension was related to “network maintenance.” A subsequent update clarified the situation, telling users the exchange had been victim to an attack and those account restrictions were necessary to prevent “unauthorized transactions.”

An official post to the exchange’s Telegram channel read,

We have detected a sophisticated and coordinated attack on specific Coinhako accounts, and have disabled the send function as a preventive measure.

The update also claimed that user private keys were not affected by the breach in security. 

Coinhako CEO Yusho Liu told CoinDesk the exchange would continue to restrict user withdrawals as a “key countermeasure against unauthorized transaction outflows” until the matter was officially resolved. User passwords and two-factor authentication were also reset as a precautionary measure. 

While the exchange did not reveal the nature of the attack, Liu claimed affected users had already been fully reimbursed for their lost funds. 

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