Twitter Inc., the fast-growing microblogging service, was inaccessible Thursday morning, struck by a "denial-of-service" attack, the company said on its status blog.
"We are defending against a denial-of-service attack, and will update status again shortly," the company said in a post shortly before 11 a.m. EDT (1500 GMT) Thursday.
In an update to the blog post, Twitter said the site is back online, but that the company is "continuing to defend against and recover from this attack." The site will remain slow for users attempting to access it while Twitter attempts to recover.
"We're working to get back to 100% as quickly as we can," the company said
Not surprisingly, posts related to the cyber attack have soared to the top of Twitter's most popular topics queue now that the site is back online.
Social-networking giant Facebook Inc. also had network issues this morning, with some users reporting that certain features have been slow or not working.
Facebook said no user data was at risk and that the problems are now mostly resolved. The site is still monitoring the situation.
An attempt to use Facebook was met with some slow services, as well as several error messages.
Denial-of-service attacks are a common weapon employed by cyber criminals to disrupt the working of Web sites. Perpetrators enlist millions of computers to attempt to access a particular site. The site cannot handle the massive increase in traffic, and is rendered inaccessible.
While disruptive and hard to trace, this type of cyber attack is considered by experts to be a relatively unsophisticated technique. The attack itself doesn't attempt to infiltrate the internal operations of a company's computer infrastructure. It simply renders its Web site inactive.
The attack on Twitter represents the latest in a string of denial-of-service attacks against several high-profile Web sites. In early July, a number of U.S. government Web sites, as well as the Web site of the New York Stock Exchange and several South Korean sites, were the target of similar actions.
Internet start-up Twitter's microblogging service has been gaining popularity among users around the globe, enabling members to communicate using short text messages up to 140 characters in length.
Still, Twitter and its founders, Evan Williams and Biz Stone, have struggled to keep pace with its explosion in popularity.
Twitter suffered several outages as the site grew in 2007 and into 2008, but has reduced such downtime over the past year. In 2007, Twitter's site was down for a total of 5 days and 23 hours, according to Pingdom, which monitors company Web sites. In 2008, that number was reduced to nearly 3 1/2 days, Pingdom said, and 84% of that downtime came during the first half of the year.
The vast majority of its employees are still focused on maintaining the service. Twitter now has more than 30 million users, up from less than 2 million a little over a year ago.
Twitter has no more information about the attack than what has been posted on its blog, which it will continue to update, the company said in an email.