Bitfinex, Hong Kong-based Bitcoin exchange operated by iFinex Inc. (Bvi), was hacked on Friday and has warned users to suspend bitcoin deposits until the potential compromise has been resolved. What’s known so far back the hack is that hackers accessed the exchange’s hot wallet and stole approximately 0.5% of the company’s total held bitcoins.

Shortly after the attack, Bitfinex released an urgent message to customers:

Dear Customer although we keep over 99.5% of users’ BTC deposits in secure multisig wallets, the small remaining amount in coins in our hot wallet are theoretically vulnerable to attack. We believe that our hot wallet keys might have been compromised and ask that all of our customer cease depositing cryptocurrency to old deposits addresses. We are in the process of creating a new hot wallet and will advise within the next few hours. Although this incident is unfortunate, its scale is small and will be fully absorbed by the company. Thanks a lot for your patience and comprehension. Bitfinex Team

Since the Bitfinex hot wallet is known, Reddit users have been attempting to guess exactly how many bitcoins have been usurped. Following the hack, Reddit user gowithbtc tracked an estimated 1474 BTC (approximately $356 thousand USD) funneled away from Bitfinex in approximately eight hours.

The year 2015 has seen a rise in the number of reported hacks against Bitcoin exchanges with five hacks occurring in February that ended in the suspension or closure of exchanges including: BTER.com, Exco.in, CAVIRTEX, Vault of Satoshi, and Netagio. In March, bitcoin and altcoin exchange Cryptoine went offline due to a hack.

While some hacks have seen the entire exchange evaporate; some have survived and continued such as BTER.com. CAVIRTEX was bought by New York-based exchange Coinsetter after shutting down.

In most cases, hot wallets of exchanges are compromised. A hot wallet is a bitcoin wallet that is accessible and open to computers in order to facilitate trades. To provide better security, most exchanges keep a large bulk of bitcoins in a cold wallet, which is a bitcoin wallet secured cryptographically and designed like putting money in a safe. Cold wallets are only accessed to replenish the stores held in hot wallets.

According to the market index at BitcoinAverage.com, BitFinex is the number two bitcoin exchange by volume with more than 25% of total. This puts the exchange second only to China-based OKCoin at 32% and above BTC-e (11.6%) and Coinbase (10.6%).

High volume exchanges especially need good security as the bigger volume, the larger the hot wallet needs to be in order to maintain trades. Early exchanges suffered a great number of hacks and many still need to better shore up their defenses.

Bitfinex also stated that the company will absorb the costs of the hack, customers will remain unaffected. A new hot wallet is also being synchronized and deposits will be re-enabled again soon.