Twitter went offline for at least two hours this morning due to a cyber attack, a high-profile target in what may be a wave of recent attacks. As of 9 a.m., the company said its site was back up but that it was “continuing to defend against and recover from this attack.” Service appears to still be very intermittent. Twitter, of course, is famous for its service issues, although lately they’ve been the result of too much (non-malicious) traffic or human error.

There appears to have been an upswing in the number of denial-of-service attacks recently, with a mass of attacks aimed at U.S. sites, including the New York Stock Exchange, The Washington Post (NYSE: WPO), and Yahoo (NSDQ: YHOO) Finance in early July. Gawker was also targeted earlier this week. And there were reports (via PC World) that there were outages at Facebook and Live Journal this morning. Unclear if these are all related.

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