Google began warning users today of its Gmail online email services when it suspects they may be targets of "state-sponsored" attacks.
It was the second time in the last two weeks that Google has deployed security-related alerts to a small fraction of those who use its services.
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But the company was coy about how it knows whether a specific individual has been targeted by attacks paid for or designed by governments.
"You might ask how we know this activity is state-sponsored," said Eric Grosse, Google's vice president of security engineering, in a Tuesday blog. "We can't go into the details without giving away information that would be helpful to these bad actors."
The new warning states: "We believe state-sponsored attackers may be attempting to compromise your account or computer." It will appear at the top of the Gmail page if the user has logged in with his or her Google account. The message is not limited to those who use Google's own Chrome, but will pop up in any browser.
Grosse was equally vague about what might trigger the alert.
"It does not necessarily mean that your account has been hijacked. It just means that we believe you may be a target, of phishing or malware for example, and that you should take immediate steps to secure your account," he said.
But it seems Google knows, or thinks it knows, a state-sponsored attack when it sees one.
"Our detailed analysis -- as well as victim reports -- strongly suggest the involvement of states or groups that are state-sponsored," Grosse claimed.
Google is in a better position than most to know.
More than two years ago Google was one of several Western companies victimized by Chinese hackers -- a rumpus that led it to relocate its search servers to Hong Kong -- and the company has cleaned up several large-scale phishing and hacking campaigns directed against Gmail users, including one in 2011 that targeted senior U.S. government officials and another later that year that affected hundreds of thousands of Iranian users.
Google has displayed similar warnings before today's.
Two weeks ago, for example, Google began alerting users whose Windows PCs or Macs remain infected with the DNSChanger malware. Those users face the loss of their link to the Internet on July 9, when authorities switch off substitute DNS (domain name system) servers that took the place of criminal-controlled machines shut down last year.