Many companies could be having their bytes burgled without realising it. This is the warning security analysts were giving in the wake of last week's discovery that Microsoft had been hacked using an old Trojan horse program.

"If a company like Microsoft is a victim of this imagine how many companies out there with fewer security measures in place have Trojans sitting on their networks," said Anna Johnson, head of Australian computer consultant firm Shake Communications. "It is highly likely that many companies are already having confidential information stolen only they don't know it," she warned.

This is just as relevant to small businesses where vital and confidential information is kept on PCs, which are in turn being more widely used online.

"If Microsoft used up-to-date anti-virus filtering on their firewall and computer systems [the attack could have been stopped]," Johnson said.

Microsoft said no source code was modified or corrupted. However it has not said whether it will pull the projects affected from development.

Anti-virus researcher Mikko Hypponen, of Finnish security firm F-Secure, said the Qaz Trojan makes it easy for an outsider to gain access to confidential data.

"We've been forecasting that worm-based industrial espionage would happen for quite some time, and it look like now it has happened big time," he said.

Microsoft said it will be working closely with US law enforcement authorities to investigate the incident and will take appropriate action when the responsible person(s) have been identified.