The blockchain is fairly secure for the most part. However, it is not completely secure. There are many different types of hacks that can be carried out by hackers. For example, Sybil attack, Routing attack, Direct Denial of Service and so on. 51% attack is also a prominent attack that can be used by hackers to steal information or coins. Other vulnerabilities include on how systems or platforms are implemented that leaves it vulnerable. Decentralized Autonomous Organization(DAO) are vulnerable to attacks.
