Basically, either the exchange itself is directly attacked, so that data can be stolen there. The biggest known incident was the hacker attack on Mt.Gox about 4 years ago, when a huge hack of Bitcoins was looted. This was also the reason for the decrease of Bitcoins 2014.
Today, the exchanges have improved to prevent such occurrences, because it can come to hacks and the trade is severely restricted or even stopped completely. This often leads to strong price fluctuations, mainly caused by so-called panic sells.
An equally large attack surface offer individual mistakes that can easily be prevented. Individual mistakes are the responsibility of individual users and are often:
- insufficiently secured user accounts due to weak passwords
- Login on a phishing site
- send the private keys to a scammer
If a stranger has once gained access to your own accounts, he can quickly transfer the entire balance to another wallet and due to the anonymity, it is no longer clearly comprehensible. For such a kind of total loss helps therefore only a careful handling with your personal user data.
But as long as you follow to the usual security rules and pay attention to secure passwords, which you should also change from time to time and do not use the same password on many other pages, an accident normally cannot happen.
The high demand and high competition between exchanges has also contributed to the improvement of safety standards.
How make you sure to safe your Bitcoins?
It is not how safe is bitcoin it is how safe your bitcoin to the others. There are many people who know what is bitcoin and what is the advantages of things that can do with it. Some people who don't have bitcoin find a people who have bitcoin and they will make a move to take or get it from him/her. If you have bitcoin make sure that your bitcoin wallet have a strong security and no one knows your password unless if you have a friend or someone who is very close to you and your trusting them.