Crypto exchanges (where cryptocurrencies are traded) are attractive targets for hackers. There have been dozens of hacks carried out throughout their history that have totaled in billions of dollars stolen (at the time of the hack).

One of the most well known and controversial hacks is the Mt Gox hack in 2013 where 850,000 BTC were stolen at once. At the time this was worth $45 million, but at BTC's peak in 2017 that would have totaled $1.7 billion. The CEO of Mt Gox was arrested in 2015 when it was discovered he had $2 million worth of BTC that had allege gone missing in the hack.

The biggest exchange hack in terms of dollar value at the time of the hack is when Japanese exchange Coincheck had 523,000,000 NEM (ticker:XEM) stolen at once, totally $533 million worth of the asset.

The hacks are generally successful because of poor security measures used by the exchange. For example, it's common practice now to store the majority of an exchange's assets in "cold storage" (wallets that have no connection to the Internet) which makes them impervious to hackers. Many exchanges, including Mt. Gox kept all their crypto in "hot wallets" (ones connected to the Internet). Other methods including phishing scams, exploiting weak points in withdrawal systems, viruses, hackers changing server code to alter users' withdrawal addresses, and many more.

Unfortunately, until recently many exchanges had no insurance for hacks. Nowadays, exchanges like Binance have started putting away a percentage of its profits to insure against hacks. Or they purchase outside insurance.

For a comprehensive list of all crypto exchange hacks, check out this article: https://selfkey.org/list-of-cryptocurrency-exchange-hacks/