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.