(Adds confirmation that UgNazi attacked Twitter)
By Gerry Shih and Joseph Menn
SAN FRANCISCO, June 21 (Reuters) - Two service outages within the course of several hours rocked microblogging platform Twitter on Thursday, as users worldwide reported significant down-time and slow service across both Twitter's website and mobile applications.
In a tweet issued shortly after 12:17 p.m. PDT (1970 GMT), the San Francisco-based company blamed the outage - one of its most severe episodes in recent months - on a "cascading bug" in one of its infrastructure components.
Twitter issued the statement after UgNazi - an emerging hacker outfit that recently gained publicity for breaking into Cloudflare Chief Executive Matthew Prince's personal Google email account - claimed credit for the service disruption in an email to Reuters, saying it launched a distributed denial-of-service (DDoS) attack against Twitter because of the company's support for the Cyber Intelligence Sharing and Protection Act.
One security professional said the group probably used a DDoS-for-hire site to launch an attack against Twitter on Tuesday, but downplayed the likelihood the group was solely responsible for bringing down the social media network.
"It was mere coincidence," the security professional said. "The backend of Twitter is having issues, which is unrelated to the very small attack."
North American traffic levels for Twitter.com sharply plummeted on two occasions between 8:30 a.m. PDT (1530 GMT) and 11:00 a.m. PDT (1800 GMT), according to data provided by network analytics company Sandvine.
The first outage lasted between 8:30 a.m. (1530 GMT) and 10:00 a.m. (1700 GMT), data showed.
Twitter acknowledged the disruption in a mid-morning blog post that was continually revised as the service resumed, only to fail for a second time before 11:00 a.m.
Tuesday's sustained outage leaves a fresh bruise on a service that had supposedly shed its unreliable reputation long ago.
As the service resumed on Tuesday, its most dedicated users quickly hopped back on to crack jokes, express relief and complain about the interruption - and the fact that, during the outage, they had nowhere to complain about the interruption.
Founded in 2006, Twitter was plagued in its early years by frequent outages as its servers struggled to handle the ever-rising volume of tweets generated worldwide, leaving frustrated users with its famous "fail whale" error screen.
The company, which has been under pressure to demonstrate a viable business model, has emphasized improving its reliability in recent years.
CEO Dick Costolo said this month that Twitter now has 140 million active monthly users. (Reporting By Gerry Shih and Mauro Whiteman; Editing by Jeffrey Benkoe, Tim Dobbyn and Andre Grenon)