Users of the online job site Monster could be at risk of identity theft and phishing attacks after it was revealed that hackers had targeted the site.

Monster has advised users, of which around 4.5m are based in the UK, to change their passwords and be aware of potential phishing email attacks after the site was infiltrated by hackers on Friday.

The attack also affects users of the USAJobs federal site (usajobs.opm.gov), which uses Monster.com as a technology provider.

It is not the first time that Monster has been the victim of hacking attacks, with 1.3m details stolen in 2007 and a another 1.6m users having their information stolen last year. That data was then used to send phishing emails and distribute malware.

Scammers are aware that phishing emails are more effective when they include personal details (as opposed to “Dear Customer” style emails), which is why recruitment websites in particular are often targeted.

In a statement, Monster senior vice president Patrick Manzo reiterated the warning to users.

“We recently learned our database was illegally accessed and certain contact and account data were taken,” said Manzo.

He went on to admit that hackers had stolen user names, passwords, telephone numbers and e-mail addresses, alongside demographic data, birth dates, gender and ethnicity, although CV’s had not been accessed.

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