Twitter was hacked last week for the third time this year, a worrisome event and another sign of the vulnerability of the social networking site that is now used by businesses to communicate with customers and conduct transitions.

The attack Friday represented a basic but unsettling breach, with the hackers getting the password to enter the master directory of Twitter's Internet addresses and then redirecting users to an alternate site instead. When users tried to reach Twitter's pages, they were sent to a site for the "Iranian Cyber Army," which claimed responsibility for the attack.

In a blog post Friday afternoon, Biz Stone, a co-founder of Twitter, confirmed that the hijacking occurred. "The motive for this attack appears to have been focused on defacing our site, not aimed at users," he said. "We don't believe any accounts were compromised."

Dan Kaminsky, director of penetration testing with the security firm IOActive, told the New York Times that "in terms of this sort" of domain name server attack, "this is easily one of the most common hacks."

Roel Schouwenberg, a senior antivirus researcher at Kaspersky Lab, an Internet security company, said the latest incident was an embarrassment to Twitter. "Even if it was the fault of the hosting company, Twitter has a track record this year of having weak passwords and being compromised," he said.

The attack should send a signal to Twitter to get its house in order.

In July, TechCrunch published internal Twitter documents that had been stolen by an unidentified hacker who figured out an employee's email password. In August, unidentified attackers bombarded several social networking sites, including Facebook and YouTube with millions of junk email messages.

For more on Twitter's problems:
- see this New York Times article

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