After penetrating a system through some vulnerability, Stewart says, the Trojan installs itself to the system and tries to phone home to a control server. Once it's connected, it can gather files and information about the network, and even take control of local systems.

Some researchers have suggested that the recent attacks were likely similar to "GhostNet," a cyber-spying operation originating in China that was said to have targeted the Dalai Lama and other human-rights activists. For that series of attacks, hackers sent target users carefully crafted e-mails containing personal information in an attempt to convince them to click a malicious link or open an attachment loaded with malware.

Last week, the security company McAfee released news that a flaw in Microsoft's Internet Explorer had opened the door to installing malware on some of the affected networks. Microsoft also issued a patch yesterday to close this flaw.

But some researchers have said that it remains unclear exactly how the company networks were attacked. Evgeny Morozov, a Yahoo! fellow at Georgetown University's E.A. Walsh School of Foreign Service, says there is no entirely coherent explanation of events. The flaw in Internet Explorer alone would not have provided complete access, Morozov says. He notes that there were likely many other important features of the attacks, including how networks and files were configured. Some have even speculated that the attackers could have had help from workers within Google.

Amichai Shulman, CTO of Imperva, a data-security company based in Redwood Shores, CA, agrees that too much attention has been placed on the flaw in Internet Explorer. "Most botnets and malware don't rely on a single vulnerability for infection," he says. "They usually try to exploit two or three vulnerabilities at the same time."

Even if Google pulls its operations out of China, it will still face Internet security threats, Morozov says. Revealing the cyber attacks may have given the company U.S. government support and a way out of a difficult censorship situation, he says, but "cyber attacks have become a daily nuisance that every company has to deal with. As long as Google offers important services like e-mail, it will still be a target."