Sorta, kinda, immutable.
Turns out the blockchain CAN get hacked, and changed.
In the blockchain, a miner who somehow gains control of a majority of the network’s mining power (a so-called “51% attack”) can defraud other users by sending them payments and then creating an alternative version of the blockchain in which the payments never happened.
For popular blockchains, attempting this sort of heist is likely to be extremely expensive. But the price gets more manageable for smaller chains and thus 51% attacks are becoming more popular.
Blockchains are also vulnerable to additional security weaknesses in the form of “smart contract bugs”. Solutions are currently being developed in the form of:
- monitoring of transactions to detect suspicious activity
- scanning smart-contract code for known vulnerabilities
- auditing services based on an established computer science technique called formal verification