Version 0.9.0 of the "Bitcoin Core" software includes five significant changes to help prevent attacks on the currency. The weakness which appears to have led to the Mt. Gox debacle saw malformed transactions similar, but not identical, to legitimate transactions already processed fool the bitcoin software. In turn, those illegitimate records didn't sync with the official bitcoin blockchain, and funds were then incorrectly calculated. Now, though, those mutated transactions can no longer be relayed or mined, and discrepancies such as mutations or doubles-spends within wallets are now reported immediately. Bitcoin explains:
"If you are running an older version, shut it down. 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."
If you love your bitcoin, you better go do just that. [Ars Technica]