In addition to having lost their jobs, Monster.com job seekers may have lost some of their personal information to cyberthieves.

Monster reported Friday that cybercriminals had breached its database and stolen contact and account data, including user IDs, passwords, e-mail addresses, names, phone numbers, and demographic data. This last category includes date of birth, gender, ethnicity, and in some cases, the state where the person resides.

A similar warning was posted on USAJobs.gov, a federal jobs Web site that depends on Monster as a technology provider.

A spokesperson for Monster declined to cite the number of affected accounts, raising the possibility that every Monster user could be affected. She said the number of users of the site is proprietary business information, as if revealing the breach total would reveal the total number of site users.

The possibility that most of site's users could be affected is consistent with the company's decision to notify users of the breach via a posting on its Web site rather than an e-mail message. "We thought it was prudent to notify them all," the spokesperson said, adding that certain job seekers outside the United States, such as those using its Jobpilot service, are not affected by the breach.

At the same time, one of the rationales for not notifying users via e-mail, the spokesperson said, is to not provide a template that could be reworked by phishers for subsequent data theft. With Monster contact information in hand, phishers could leverage public worry about the breach among account holders to solicit still more information under false pretenses.

Patrick Manzo, senior VP and global chief privacy officer of Monster, said the thieves did not access resumés stored with the site and that sensitive data, such as Social Security numbers or financial information, was not stolen because Monster does not collect such data.

"Immediately upon learning about this, Monster initiated an investigation and took corrective steps," he said in a statement. "It is important to know the company continually monitors for any illicit use of information in our database, and so far, we have not detected the misuse of this information."

The last time the company announced corrective security measures was in August 2007, when -- following the theft of contact details from 1.3 million accounts by malware known as Infostealer.Monstres -- the company committed itself to "institute a comprehensive set of new systems and processes designed to enhance existing security and minimize such threats in the future."

Two months later, the new systems and processes failed to stop an iFrame from being injected into portions of Monster. The iFrame attempted to redirect visitors to the affected Monster pages to a malicious site.

Monster and USAJobs both recommend that account holders change their passwords immediately. Both sites note that a password change may soon be required. Monster is working with law enforcement authorities to investigate the incident, a spokesperson said.

"We continue to devote significant resources to ensure Monster has appropriate security controls in place to protect our infrastructure, and while no company can completely prevent unauthorized access to data, Monster believes that by reaching out to job seekers, the company can help users better defend themselves against similar attacks," said Manzo.

Last week, Heartland Payment Systems warned that it had been the victim of malware. The size of the resulting data breach remains undisclosed. But based on the number of transactions handled by the company, the Heartland breach could be one of the largest on record.