The DAO contract (at
0xbb9bc244d798123fde783fcc1c72d3bb8c189413) was hacked in 2016, but is still operational and currently holds over 43 Ether:
Moreover there are still often transactions made to it, several per week on average. Why isn't the DAO being exploited again? During the hardfork was a change made so that the vulnerable function in the contract
splitDAO couldn't be called anymore?