"We are defending against a denial-of-service attack, and will update status again shortly," the Twitter status page post said at around 10:50 a.m., leaving its 20-plus million users unable to post their tweets.
The site was restored a few minutes ago and has been up and down, and Twitter officials say they are still under attack -- and battling it.
"We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate," Twitter co-founder Biz Stone said in a new blog post after the site was restored.
Twitter from time to time experiences server slowdowns and server unavailability in what its community calls the Twitter "fail whale," but nothing as large-scale as this outage.
"This is the first [DDoS] on Twitter I know of. I am sure there have been attempts before," says Richard Stiennon, chief research analyst for IT-Harvest, who also posted a blog on the DDoS this morning.
Facebook users also saw some slowdowns, as Twitter users apparently went to Facebook to find out what was going on with Twitter, Stiennon said in his post.
But a new report says Facebook and LiveJournal have also acknowledged DDoS attacks on their sites as well.
Paul Henry, security and forensics analyst for Lumension Security says Koobface is likely a big part of the equation: "Today's outage is happening at the same time a new version of the Koobface malware has been found in the wild that is using both Twitter and Facebook messages to send invitations that are designed to lure potential victims to fake AV Web pages. The speculation is that the onslaught of bogus messages that are directing users to malicious pages may in fact be overwhelming Twitter," Henry says.
Graham Cluley, senior technology consultant for Sophos and a prolific Twitter user, says the outage could affect plenty of businesses that are increasingly using Twitter to communicate with their peers. "This will impact more and more people, and not just the ones standing in line waiting for their coffee [and tweeting]. People use Twitter for work now...and their work day will be disrupted."
The culprit of such a powerful attack is most likely a botnet, experts say.
Cluley says that, meanwhile, the Twitter community can help by checking the security of their own machines. "If you want to your little bit, scan your computer with an updated antivirus and make sure you're properly patched so that you're not passing a DDoS to Twitter or thousands of other companies. That makes good sense," he says.
This post will be updated regularly as details unfold.
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