The Guardian has warned 500,000 people that details they posted to the newspaper's employment site may be in the hands of hackers.
The Guardian posted a warning of the breach on its Web site on Friday. On Saturday, the newspaper said the system had been secured and those affected had been contacted by e-mail.
In an attempt to play down the breach, the Guardian said it affected "only a minority" of the 10,328,290 unique users who visit the site annually, and that some of the data lost was up to two years old.
"The police remain anxious to keep information about the apparent theft to a minimum, in order not to compromise their investigations, but did agree with us that we could inform those users who may be affected," the Guardian said. "We stress our regret that this breach has occurred. This is apparently a deliberate and sophisticated crime, of which the Guardian is a victim in addition to some of our users."
A spokeswoman for the Metropolitan Police said the Police Central e-Crime Unit is investigating, but there have been no arrests.
The Guardian's Jobs Web site runs on software from a company called Madgex. The company said Monday that hackers accessed personal details from some job seekers' CVs on Friday.
Madgex said it had taken "preventative measures to ensure the same issue" doesn't affect the job boards of its other clients, which includes the Times and the Sun.
"Unfortunately, no site can ever be warranted as 100 percent safe from concerted and technologically sophisticated criminal hackers," said Madgex CEO Simon Conroy via e-mail.
On the Guardian's Jobs site, users can upload their CVs. Information in resumes and CVs could be of great use to data thieves, since those documents may contain e-mail addresses, postal addresses, job histories and a wealth of other personal information. The data could be used for identity fraud.
In January, a database for the jobs site Monster.com was illegally hacked. The stolen data included user IDs and passwords, e-mail addresses, birth dates, gender, ethnicity, and in some cases, users' states of residence in the U.S. No resumes or Social Security numbers were lost.
Monster was also the target of a massive data breach two years ago. Hackers obtained the log-in credentials for companies seeking employees then accessed Monster's database, sending up to 1.6 million records to a remote server.