Hackers are believed to have stolen the personal details of millions of people using the online job site Monster.
Users around the world have been affected, including the 4.5 million users of the UK site.
If all are affected it would make it the biggest data theft in the UK since the details of 25 million child benefit claimants went missing last year.
The recruitment giant has advised people to change their passwords and be on the lookout for phishing e-mails.
Recruitment sites have proved rich pickings for criminally-minded hackers in the past and it is not the first time Monster has fallen foul of cyber thieves.
In 2007, 1.3 million details were downloaded to servers based in Ukraine.