One approach is that contracts can be associated with their hash. It is a bit silly for everyone to be writing the same smart contract over and over, with some

One approach is that contracts can be associated with their hash. It is a bit silly for everyone to be writing the same smart contract over and over, with some versions being erroneous or malicious. So instead of having to run a potentially malicious contract, you are given a hash that points to one of a few different smart contracts that are known to be safe. The community has "white listed" a bunch of these known contracts and their hashes, so you are relatively safe using any contracts in these lists. Most miners in various cryptocurrencies will only run code that is on their communities version of the whitelist. Of course this only helps with some issues, not all. *edit: and this is not ethereum specific. This approach was one way bitcoin tried to solve the problem and it has spread to most other cryptocurrencies I believe.