All you need in order to be exploited is to be using software with 0day exploits. Many known exploits are not public. In a very real sense, you are only protected to the extent that you are a small target.
As the potential payoff of a hacker approaches $1 million, the likelihood of being hacked approaches 90%. Software really is THAT insecure and bitcoin thefts are not prosecuted making it basically risk-free to steal bitcoins.