The WordPress blog-hosting service has been hit by a denial of service (DoS) attack. The attack started last Saturday and was was still affecting sites last night.

Matt Mullenweg, spokesman for Automattic, confirmed that the service had experienced a DoS attack with spikes of up to 6 gigabits of incoming traffic, which was making some blogs inaccessible for about five to 15 minutes. Though service had mostly been restored, Automattic, which maintains WordPress.com, was still working on returning service to normal levels, he said.

"Obviously that [is not good] and is pretty unusual for our service," he said. "All our people who can are working on the issue."

However, an employee at a New York-based company that has blogs hosted by WordPress.com suggested that some users were experiencing outages for longer than 15 minutes. The source, who asked not to be identified, said on Tuesday afternoon that users there were unable to log in to their blogs and post comments for "most of the day." However, the blogs were still able to be viewed publicly.

"It's starting to come back to life now, slowly," said the source yesterday.

WordPress users were notified via email about the DoS attack. In the email, the service provider said that the attack was affecting user log-in and causing some forums to be offline.

Mullenweg said that the main Wordpress home page was down longer than some blogs because "we sacrificed it in order to keep blogs and our users up." He also provided a link to a graph that shows the traffic spikes to WordPress.com on the graph, where the service's traffic is displayed in a brown line.

A DoS attack is an attempt to make a website or service unavailable to intended users by flooding the service or site with incoming data requests, such as emails. Motives for DoS attacks vary, but perpetrators mostly target companies with high-profile, highly trafficked websites.

Joris Evers, a spokesman for security research and software company McAfee, said DoS attacks were still fairly common, although they have tapered off in recent years because technology has been developed that can head off such attacks before they affect service.

Though he had not heard specifically of the WordPress attack, Evers said that it's possible the attack was mounted by someone "who was upset about something that was written on a WordPress blog, and they decided to take action against that."