Although the attacks against Marathon Oil, ExxonMobil, and ConocoPhillips occurred in 2008, the companies didn't find out about them until sometime in 2009. According to the Christian Science Monitor, sources say hackers tried to extract data that included e-mail passwords, messages, and other information tied to executives with access to proprietary exploration and discovery information.
There is no confirmation that China is responsible. However, data from at least one of the compromised computers was found to be transmitted to a computer based in China. In addition, the breaches are being referred to as being caused by the "China virus."
News of the attacks come in the wake of recent cyber attacks on Google and several other companies that apparently originated in China.