The problem was caused by a denial-of-service attack, which means that the website was immobilized by information requests flooding the network. This left customers of two major exchanges: Bitstamp, a Slovenia-based trader, and Blockchain, a UK-based trader unable to withdraw their funds. Many other exchanges have experienced delays in their transactions.
The hack targeted what is called transaction malleability, or the possibility of transaction messages being renamed. The problem is that the bug makes it possible for transactions to look like they did not go through, when they actually did. This leaves the potential for Bitcoins to be re-sent when the recipient has already received the proper amount of currency.
Bitcoin Foundation’s chief scientist, Gavin Andresen, said that the hack is due to both the exchanges software and the main Bitcoin software. He also said that the person behind the attack is not stealing anything, but is preventing some transaction confirmations. Andresen stressed the importance that no one’s Bitcoin wallets are being affected.