The software driving Bitcoin transactions on the Internet has been updated to fix a weakness that contributed to the downfall of Mt. Gox, once one of the biggest exchanges for the digital currency.

Version 0.9.0 of the "Bitcoin Core" software , the Bitcoin infrastructure software previously known as Bitcoin-QT, contains five separate changes designed to make so-called transaction malleability attacks harder to pull off. As Ars explained last month , the attacks work by flooding exchanges with large numbers of malformed transactions that are similar, but not identical, to legitimate transactions that have already been made. Exchanges that trust one or more of the phantom records instead of the entries in the official Bitcoin blockchain can fall out of sync with the rest of the network and must recalculate their fund balances once the mistakes become apparent.

Attacks that abused the weakness caused several exchanges to suspend cash withdrawals. Tokyo-based Mt. Gox never recovered. Three weeks ago, it filed for bankruptcy after claiming to lose $468 million, $412.5 million of which it said belonged to customers.

In version 0.9.0, the transaction malleability weakness has been fixed by tightening transaction rules preventing “mutated transactions” from being relayed or mined. It also contains new functions that report wallet transactions that conflict with each other or that contain incorrect balances for double-spent (or mutated) transactions. It also includes an installation executable that works on 64-bit versions of Windows.

"If you are running an older version, shut it down," Wednesday's release notes stated. "Wait until it has completely shut down (which might take a few minutes for older versions), uninstall all earlier versions of Bitcoin, then run the installer (on Windows) or just copy over /Applications/Bitcoin-Qt (on Mac) or bitcoind/bitcoin-qt (on Linux). If you are upgrading from version 0.7.2 or earlier, the first time you run 0.9.0 your blockchain files will be re-indexed, which will take anywhere from 30 minutes to several hours, depending on the speed of your machine. On Windows, do not forget to uninstall all earlier versions of the Bitcoin client first, especially if you are switching to the 64-bit version."