Twitter was unavailable early Thursday morning due to a denial of service attack.

In a blog post, presumably hosted on an unaffected server, Twitter co-founder Biz Stone acknowledged the attack. "Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users," he said. "We are defending against this attack now and will continue to update our status blog as we continue to defend and later investigate."

A denial of service attack involves bombarding a Web site or server with more traffic than it can handle, effectively causing online gridlock. Often such attacks are distributed, meaning that multiple computers, usually compromised by malware, send data to the target site in unison.

Twitter was knocked offline about 9:15 a.m. EDT.

Pingdom, a site that tracks server uptime, indicates that Twitter was offline for about two and a half hours.

The Twitter status page says that Twitter availability has been restored. The site's last significant unplanned downtime occurred on May 30 following a software error.

According to Netcraft, Twitter runs on an Apache Web server hosted by Verio.

With the restoration of Twitter, tweets have resumed, many of them about the outage.

Denial of service attacks are relatively common. Earlier this week, Gawker Media sites were hit with a denial of service attack. And last week, AT&T blocked notorious Internet forum 4chan because, it said, the site was undergoing a denial of service attack.

Last week, Internet Systems Consortium warned that BIND 9, the most common domain name server, contained a vulnerability that could be exploited to crash the server, leading to a denial of service. The company said that the vulnerability is actively being exploited and it urged users of the software to update immediately.

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