Twitter a target of outside attack

Twitter, the fast-growing social media for trading short messages, suffered a prolonged attack Thursday, knocking the service out for hours in the morning and resulting in slow and intermittent service later in the day.

The company said it was the victim of a "distributed denial of service," or DDoS, attack, a disruptive tactic in which outsiders attempt to overload a Web site with huge numbers of computer-generated requests — even, for instance, to refresh a Web page.

Facebook, whose users encountered intermittent problems Thursday morning, was also the subject of a denial-of-service attack, though it was not known whether the same hackers were involved. Unlike Twitter, Facebook never became completely inaccessible. Facebook said no user information was at risk.

Because sites have a limited capacity to handle such requests, a deliberate deluge can cause servers to respond slowly to legitimate users or not at all.

"It basically just shows that Twitter wasn't spending the money to filter out DDoS attacks," said John Pescatore, a security analyst at research company Gartner. Because Twitter is a consumer-oriented site, he said, it's common that such sites learn the hard way because they "don't invest in the reliability and the protection against threats that a business-grade service does."

Twitter's traffic and membership have exploded in the past year, rocketing to more than 20 million unique visitors in June from 600,000 a year earlier, according to Web- ratings company ComScore. And Nielsen's Web ratings service said Twitter reached nearly 10 percent of online users in June.

"Their popularity and their utility are beginning to make them a target for a lot of different kinds of malicious activity," said Sarah Milstein, co-author of "The Twitter Book." "You hate to say a denial-of-service attack legitimizes a site as a serious business, but I think in some ways it's a significant milestone."

Twitter came partially back online later in the day, but users reported spotty performance and many said they could not access the service on mobile devices such as iPhones and BlackBerrrys.

"We're working to get back to 100 percent as quickly as we can," said a post on the company's site. But the site was still experiencing slowness nearly 12 hours later.

Twitter's power to quickly and widely circulate short messages of up to 140 characters has been visible in many segments of business and media.

"For the people who use it, Twitter is becoming a primary alert system within the information economy, an editor of the live Web," said Jay Rosen, a media professor at New York University. "When that alert system is down, it feels like the Web is broken."

The site has played a role in several high-profile news stories, including the recent upheaval over elections in Iran, where observers used the service to transmit goings-on on the ground.

Before that Twitter was a source of information about the terrorist attacks in Mumbai, India, and the origin of the first picture of a plane that landed in New York's Hudson River this year.

"I had to Google-search Twitter to find out what was going on, when normally my Twitter feed gives me all the breaking news I need," said Alison Koski, a New York public-relations manager.

Inevitably, the terms "DDoS" and "denial-of-service" quickly emerged as the top "trending topics" on Twitter and reverberated among the digerati.

"Oooh, dramatic!" declared Mike Arrington at TechCrunch. "That's way more exciting than run-of-the-mill outages that have plagued the site since forever."

Caroline McCarthy, of CNet News, concurred in her column, The Social: "Way back when, Twitter outages were so commonplace that it was worth reporting when it didn't crash — as when it stayed afloat during the entire South by Southwest Interactive Festival in 2008. Now, a few million dollars of venture capital later, the service is far more stable.

"It's been a crucial platform for information exchange in the face of global events where more traditional means of broadcasting have been inaccessible or blocked."

Information from The Associated Press and San Jose Mercury News is included in this report.