That is what the FBI is probing, although Citibank claims ‘all is well’.

The Federal Bureau of Investigation is probing a computer-security breach targeting Citigroup Inc. that resulted in a theft of tens of millions of dollars by computer hackers who appear linked to a Russian cyber gang, according to government officials.

The attack took aim at Citigroup’s Citibank subsidiary, which includes its North American retail bank and other businesses. It couldn’t be learned whether the thieves gained access to Citibank’s systems directly or through third parties.[...]

[...]The Citibank attack was detected over the summer, but investigators are looking into the possibility the attack may have occurred months or even a year earlier. The FBI and the National Security Agency, along with the Department of Homeland Security and Citigroup, swapped information to counter the attack, according to a person familiar with the case. Press offices of the federal agencies declined to comment.[...]

[...]Joe Petro, managing director of Citigroup’s Security and Investigative services  “I’m not aware of any major case the FBI is working in our company at this magnitude.”[...]

[...]Security officials worry that, beyond stealing money, hackers could try to manipulate or destroy data, wreaking havoc on the banking system. When intruders get into one bank, officials say, they may be able to blaze a trail into others.[...] (Source: WSJ)

Interesting, the FBI and Citibank were swapping information to counter the attack, yet CitiBank says no attack took place. I guess it would not look good if the nations worst bank, which is being propped up by the tax payers was hacked and money was stolen. So just sweep it under the rug CitiBank, like you do with all your other losses.

I wonder, if CitiBank was hacked and money was stolen, would they value the losses at mark to market? Sarcasm intended