DDoS (distributed denial-of-service) is a common type of hacker attacks whose gist boils down to firing a large number of similar queries at a network. Bitcoin boasts built-in defenses against this attack vector. In order to avert the congestion of nodes memory, the block size is restricted to 1 MB and the size of a script cannot exceed 10,000 bytes. Additionally, each block cannot request more than 20,000 confirmations. There is also a 20-key limit for multi-confirmations.
Furthermore, clients have the ability to block dubious nodes and transactions. For instance, the most recent edition of the Bitcoin Satoshi client keeps track of anomalous transactions the size of which is greater than 100 kilobytes. As part of the transaction processing routine, this client also makes sure that the outputs aren’t spent.