Hackers claim to have compromised 900,000 blogging accounts hosted by the Livejournal blogging service.
Livejournal has not confirmed the number of accounts compromised by the hacking gang, known as Bantown, but it says it has changed its security systems to combat any threats.
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Livejournal is owned by Six Apart, the manufacturer of the Movable Type blogging software.
Six Apart has about two million active LiveJournal users and more than nine million registered accounts.
Cookies are small data files stored on computers that are used by websites to identify visitors and give them quick access. By obtaining cookie information, it is believed the hackers were able to hijack the Livejournal accounts.
Blogging is now a business for some of the best-read bloggers, with companies paying for adverts to be carried on the sites with the most visitors.