Instead of the usual Twitter Web site design, visitors to the site instead saw a black screen with an image of a green flag and Arabic writing. The defaced site also included a message that said, "This site has been hacked by Iranian Cyber Army," and an e-mail address.
Whether or not Iranian hackers are responsible for the attack wasn't immediately clear. However, Twitter and other Internet sites have been used by Iranian opposition groups and protestors to share details of anti-government protests in that country.
Twitter blamed the outage on changes made to the company's DNS (Domain Name System) records, which match the company's domain name with the IP addresses of its servers.
"Twitter's DNS records were temporarily compromised but have now been fixed. We are looking into the underlying cause and will update with more information soon," Twitter said on its Twitter Status page.
Based on Twitter's account of the attack, it's possible that the company's servers were never compromised. The actual attack may have instead targeted Dyn, the DNS service provider that manages Twitter's DNS records, according to whois records.
While the outage left Twitter users cut off from the service for about an hour, the type of attack wasn't serious, according to Dhillon Andrew Kannabhiran , founder and CEO of Hack In The Box, a Malaysian company that runs security conferences in Europe, the Middle East and Asia.
"Yawn, is my comment. It was a simple defacement. So what?" Kannabhiran said.