An online attack Friday morning on Twitter was the result of the simplest of security breaches: someone got the password to enter the master directory of Twitter’s Internet addresses and then redirected users to an alternate site instead.

No user information appears to have been stolen in the attack. But the security breach  the third major one at Twitter this year  underscores the continuing weakness of the company’s systems as its micro-blogging service is becoming more important to business and even global politics.

The incident also highlights a basic vulnerability in the way life is lived as it becomes increasingly digital: With so much vital information stored on the Web, people are only as safe as their passwords.

During the assault, which security analysts said began about 1 a.m. and lasted roughly an hour, hackers tinkered with Twitter’s domain name servers, which are hosted by a Manchester, N.H., company called Dyn. When Web surfers tried to reach Twitter’s pages, they were sent instead to a site for the “Iranian Cyber Army,” which claimed responsibility for the attack.