MI5's official website has been hacked in a bid to steal the identities of visitors to the site, according to media reports.
The hackers, who called themselves Team Elite, rigged the site to download viruses onto the machines of anyone using the British intelligence services website, the Daily Express reported.
The hackers were also able to discover the identity of anyone using the website, and find out every other website they had visited years earlier, the paper continued.
MI5, which is responsible for monitoring threats inside the UK, did not confirm or deny the attack, nor respond to questions on whether website visitors were affected. A Whitehall spokesperson said: "MI5 take security very seriously. Their website is secure and hosted in a high security environment."
But media reports have quoted the MI5 admitting there had been a "small issue" with a search engine linked to the MI5 website. It is believed hackers penetrated a search engine related to the MI5 website.
Graham Cluley, senior technology consultant at Sophos, wrote in his blog there were gaps in the original news report. "According to the newspaper, the hackers were only able to steal information from users visiting the site - so the only way in which I could imagine such information could have been grabbed by the cybercriminals would have been if an MI5 agent visited the site and was storing unencrypted information about fellow agents and terror suspects on their own computers."
"The report also says that the hackers were able to download viruses onto the computers of visiting users. Sadly, they haven't gone into any details as to what virus this was, so we don't know what it could have done."
But Cluley added the story should still serve as a reminder to website owners to assess the risks posed by hackers. "Website owners should take this as a warning that they are vulnerable to embarrassing hacks if they are not properly protected."
Paul Vlissidis, technical director of Secure Test NCC Group, said "While this attack is certainly embarrassing for MI5, most websites are actually ‘owned’ and operated by marketing functions. Even in security aware organisations these functions are rarely security specialists."
"Assuming that the MI5 website is subjected to regular independent security and penetration testing, it is possible was the result of a blip in the testing process or a change to the website that had been made and not yet tested, although not having the full details prevents us from speculating too much on what the issue was. There is a very small chance that the attack was due to a new vulnerability which had not yet been disclosed, although this is the least likely possibility."