Security for internet applications is a spectrum, and it would be safe to assume that any application might have unnoticed vulnerabilities waiting to be exploited. Cryptocurrencies are especially attractive targets for hackers: because the technology is so novel, it is more likely to be hiding bugs, and the code usually interacts with tokens that have real-world value. Ethereum is no exception.
Attackers have successfully stolen ether using a number of tactics, which tend to aim at Ethereum smart contracts (written in Solidity), the network itself, cryptocurrency exchanges, or end users.