Google has begun warning users of its Gmail online email services when it suspects they may be targets of "state-sponsored" attacks.
The move is 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.
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 victimised 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.
In July 2011, Google also warned customers whose systems were infected with fake antivirus software, or "scareware." In that instance, Google became suspicious when it uncovered "unusual search traffic" while doing maintenance at one of its data centers.
Grosse did not explain what event, if any, sparked Google to roll out today's warning.
But sophisticated cyber-weapons believed to be state-backed have been in the news of late.
Last week, security researchers announced they had found a sophisticated espionage tool, which they called "Flame" (and in some cases, "Flamer"). Flame pilfered vast amounts of data from Middle Eastern computers, most of them located in either Iran or Palestine.
Some experts believe that because of its size and complexity, as well as the need to digest the huge amount of data is hoovers, Flame is probably state-sponsored.
And just last Friday, the New York Times reported that President Barack Obama had ordered cyberattacks against Iran -- using the Stuxnet worm -- in an attempt to disrupt or delay that nation's nuclear fuel enrichment program.
Gmail-specific warnings are also not new. Since March 2010, Google has notified Gmail users when it suspects account hacking attempts. Google triggers that alert in part on the Internet Protocol (IP) address of each successful log-on.
Google's state-sponsored warning includes a link to a page on Google's Help website, where the company hinted at why it issued the alert.
"It's likely that you received emails containing malicious attachments, links to malicious software downloads, or links to fake websites that are designed to steal your passwords or other personal information," the help page states.
That page also repeated some of what Grosse had written.
"It's important to note that Google's internal systems are not compromised and that this message does not refer to one specific campaign," the page read. "We routinely receive abuse reports from users, as well as from our internal systems that monitor for suspicious login attempts and other activity."
Google urged users who receive the warning to update their software, including their browsers, operating systems and browser plug-ins; ensure they're logging onto the legitimate Gmail website of https://accounts.google.com//; and use Gmail's two-factor authentication.
The latter sends a second password to the user's pre-defined phone number before allowing log-on.