From an extensive blockchain reality check
"In total, hackers have stolen nearly $2 billion worth of cryptocurrency since the beginning of 2017, mostly from exchanges, and that’s just what has been revealed publicly. These are not just opportunistic lone attackers, either. Sophisticated cybercrime organizations are now doing it too: analytics firm Chainalysis recently said that just two groups, both of which are apparently still active, may have stolen a combined $1 billion from exchanges.Once hailed as unhackable, blockchains are now getting hacked | MIT Technology Review
We shouldn’t be surprised. Blockchains are particularly attractive to thieves because fraudulent transactions can’t be reversed as they often can be in the traditional financial system. Besides that, we’ve long known that just as blockchains have unique security features, they have unique vulnerabilities. Marketing slogans and headlines that called the technology “unhackable” were dead wrong."