A 700MB file that hackers claimed contains valuable database information on bankrupted MtGox is actually hiding Bitcoin wallet file-stealing malware, researchers have warned.

Kaspersky Lab’s Sergey Lozhkin claimed in a blog post last Friday that the entire data leak story, which emerged after MtGox CEO Mark Karpeles had his blog hacked, was invented to lure users into unwittingly downloading the malware.

Although the 716MB file features trades.zip, the file actually contains nothing but publically available data on MtGox trades, he said.

The real purpose of the file is Trojan malware designed to “search and steal” Bitcoin wallet files from the victim.

He continued:

We detect the Windows Trojan (MD5:c4e99fdcd40bee6eb6ce85167969348d), a 4.3MB PE32 executable, as Trojan.Win32.CoinStealer.i and OSX variant as Trojan.OSX.Coinstealer.a. Both have been created with the Livecode programming language – an open-source and cross-platform application development language. When the victim executes the application, it looks like the back-office software for accessing the databases of Mt. Gox’s owning company, Tibanne Co. Ltd.

The malware executes TibanneSocket.exe and then goes on the prowl for bitcoin.conf and wallet.dat files.

If the attackers find the latter, and they have been stored unencrypted, they will “gain access to all the Bitcoins the user has in his possession for that specific account”, Lozkhin warned.

A week ago, hackers hijacked MtGox CEO Karpeles’ blog and posted a file which they claimed had been nabbed from the company’s servers.

They said the file proved that the exchange, once the world’s largest, still controlled almost one million Bitcoins despite having just declared bankruptcy.

As interest in the exchange grows following its bankruptcy filing, MtGox has already released a notice warning former users not to fall for phishing emails piggy-backing on the case. ®