Back at the dawn of the Web, the most popular account password was “12345.”

Today, it’s one digit longer but hardly safer: “123456.”

Despite all the reports of Internet security breaches over the years, including the recent attacks on Google’s e-mail service, many people have reacted to the break-ins with a shrug.

According to a new analysis, one out of five Web users still decides to leave the digital equivalent of a key under the doormat: they choose a simple, easily guessed password like “abc123,” “iloveyou” or even “password” to protect their data.

“I guess it’s just a genetic flaw in humans,” said Amichai Shulman, the chief technology officer at Imperva, which makes software for blocking hackers. “We’ve been following the same patterns since the 1990s.”

Mr. Shulman and his company examined a list of 32 million passwords that an unknown hacker stole last month from RockYou, a company that makes software for users of social networking sites like Facebook and MySpace. The list was briefly posted on the Web, and hackers and security researchers downloaded it. (RockYou, which had already been widely criticized for lax privacy practices, has advised its customers to change their passwords, as the hacker gained information about their e-mail accounts as well.)

The trove provided an unusually detailed window into computer users’ password habits. Typically, only government agencies like the F.B.I. or the National Security Agency have had access to such a large password list.

“This was the mother lode,” said Matt Weir, a doctoral candidate in the e-crimes and investigation technology lab at Florida State University, where researchers are also examining the data.

Imperva found that nearly 1 percent of the 32 million people it studied had used “123456” as a password. The second-most-popular password was “12345.” Others in the top 20 included “qwerty,” “abc123” and “princess.”

More disturbing, said Mr. Shulman, was that about 20 percent of people on the RockYou list picked from the same, relatively small pool of 5,000 passwords.

That suggests that hackers could easily break into many accounts just by trying the most common passwords. Because of the prevalence of fast computers and speedy networks, hackers can fire off thousands of password guesses per minute.

“We tend to think of password guessing as a very time-consuming attack in which I take each account and try a large number of name-and-password combinations,” Mr. Shulman said. “The reality is that you can be very effective by choosing a small number of common passwords.”

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