The Daily Express has claimed that hackers known as Team Elite have infiltrated the MI5 site, altering it so "the identity of visitors could be stolen and viruses downloaded on to their machines."
According to the newspaper, Conservative MP Patrick Mercer said:
"Having potentially highly classified information available to hackers is deeply concerning. The identity of agents and informers in terror groups such as Al Qaeda are held by MI5."
MI5 says the problem has now been fixed.
However, in a blog post, Sophos senior technology consultant Graham Cluley pointed out that the hackers could only compromise details of site visitors, not content of the site itself.
"Frankly, there’s a lack of information in this report to let us know just how big a security problem this was. The good news is that the authorities claim to have fixed the flaw, so visitors to the site are no longer at risk."
"However, this is a timely reminder for all of us who have web sites – big and small – to make sure they are properly secured from attack."