If you're feeling paranoid, get your messages offline

Graham Cluley, Sophos

"This is a serious vulnerability with Hotmail," said Graham Cluley, senior technology consultant at the anti-virus firm Sophos.

But the process is cumbersome and involves some guesswork, limiting the threat to privacy.

"The good news is that the average person in the street doesn't need to worry, as they would have to be specifically targeted," said Mr Cluley.

"But if you're feeling paranoid, get your messages offline," he added.

Messages exposed

Hotmail is one of the world's most popular web-based e-mail services, with Microsoft saying it has more than 110 million active accounts.

"Hotmail has been notified so it might not work for much longer but it works as of right now," says a message on the hackers' website.

There is the potential for some serious damage

Craig Whitney, Internet Security Systems

The flaw only allows you to read specific messages. You cannot get access to the inbox or other parts of the e-mail account and you first need to log in to Hotmail using your own account.

"There is the potential for some serious damage," said Craig Whitney, sales manager for Europe and the Middle East at the Managed Security Services division of Internet Security Systems.

The flaw exploits the way Hotmail organises messages. Every e-mail has a consistent format and the same number of digits.

To gain access to the e-mails, you need to know a person's username and guess the number of a message.

Limited impact

To get round this long process, Root Core have devised a scanning programme that tries about one message number per second.

Mr Whitney said various factors could limit the impact of the security flaw.

He said you would need a fast internet connection to run the scanning programme and know how often someone looked at their Hotmail account.

Additionally there would be a clear trail back to the original Hotmail account used to hack another person's e-mails.

"It raises the question of e-mail as a secure way to communicate," said Mr Whitney, comparing it to sending a letter in a transparent envelope.

Microsoft targeted

Microsoft has taken the brunt of criticism for security flaws exposed over the internet.

Hackers have targeted its server software, Windows operating system, Outlook e-mail program, Internet Explorer browser, instant messaging software and Hotmail.

"The problem is that Hotmail is probably the most popular web-based e-mail service, so hackers are drawn to target it," said Mr Cluley.

"It's not necessarily that Microsoft software has more holes, but that more people are targeting their software as there is more of it."

Root Core describes itself as a group which focuses on "information sharing not causing havoc."