Code is the (F)Law: Demystifying and Mitigating Blockchain Inconsistency Attacks Caused by Software Bugs

Blockchains promise to provide a tamper-proof medium for transactions, and thus enable many applications including cryptocurrency. As a system built on consensus, the correctness of a blockchain heavily relies on the consistency of states between its nodes. But consensus protocols of blockchains only guarantee the consistency in the transaction sequence rather than nodes' internal states. Instead, nodes must replay and exe-cute all transactions to maintain their local states independently. When executing transactions, any different execution result could cause a node out-of-sync and thus gets isolated from other nodes.After systematically modeling the transaction execution process in blockchains, we present a new attack INCITE, which can lead different nodes to different states. Specifically, attackers could invoke an ambiguous transaction of a vulnerable smart contract, utilize software bugs in smart contracts to lead nodes that execute this transaction into different states. Unlike attacks that bring short-term inconsistencies, such as fork attacks, INCITE can cause nodes in the blockchain to fall into a long-term inconsistent state, which further leads to great damages to the chain (e.g., double-spending attacks and expelling mining power). We have discovered 7 0day vulnerabilities in 5 popular blockchains which can enable this attack. We also proposed a defense solution to mitigate this threat. Experiments showed that it is effective and lightweight.