Unlike financial institutions, blockchains confirm transactions only after all nodes in the network are in agreement. Until a block with a transaction is verified, the transaction is classified as unverified. However, verification takes a certain amount of time, which creates a perfect vector for cyberattacks.
Double-spending is a common blockchain attack exploiting the transaction verification mechanism. All transactions on a blockchain need to be verified by users in order to be recognized as valid, which takes time. Attackers can use this delay to their advantage and trick the system into using the same coins or tokens in more than one transaction.