A quarter of the 11 million Britons who use social networking sites such as MySpace and Facebook may be leaving themselves open to identity fraud.
Get Safe Online, a government-backed campaign group, is warning against posting personal details online.
Its research also showed eight million people leave home wireless networks unprotected against intruders.
And more than half of the over-65s polled use a single password for every website they visit, the group said.
The group is advising older internet users to change their passwords more often.
Tony Neate, managing director of GetSafeOnline.org, said a date of birth and address details were enough for someone to set up a credit card in another name.
"Although some of these details may seem harmless, they actually provide rich pickings for criminals," he said.
Cabinet Office minister Gillian Merron said the risks can be easily fixed and did not mean people should stop using social networking sites and wireless networks.
News source: BBC Technology News