The website of MI5 was breached by hackers when a search engine related to it was penetrated.
The problem was not a direct threat but it is cause for concern, according to security experts.
MI5 is the UK's counter intelligence security agency which monitors security threats in the UK.
According to a report in the Daily Express the hackers could have put viruses on the computers of people using the website and find out their identities.
But a source said there was no confidential information at risk because no sever was connected.
Paul Vlissidis, technical director at the penetration testing division of NCC Group, said it is unlikely that any public facing site such as this will be connected to servers with confidential information.
A Government spokesperson said: "MI5 take security very seriously. Their website is secure and hosted in a high-security environment"
But Vlissidis said it is cause for concern. "It is a concern because the vulnerability should have been picked up in penetration tests."
Matt Hampton, at security firm Imerja said the hack only effected information in the browser and not confidential information on the server.
"However the fact that this was not picked up by our own security service is a serious concern."
The problem has now been fixed.
Read more on Hackers and cybercrime prevention
Surveillance expert ‘unfairly’ refused job at intelligence regulator after MI5 intervened
MI5 accused of withholding surveillance compliance failures from cabinet minister
MI5 slammed by watchdog for failing to delete intercepted phone and internet data
MI5 failed to disclose failings in handling intercepted data, court hears