Wireless internet networks in millions of homes can be hacked in less than five seconds.
The wi-fi hacking means criminals can spy on the activities of families, perhaps stealing their identity and banking details to raid their accounts, says a new study.
The hackers could also use the wi-fi access to tap into illegal pornography or upload and download stolen music and movies without being traced.
An 'ethical hacking' experiment in six cities, using freely available software, found almost 40,000 home wi-fi networks at high risk, reports the Daily Mail.
Separately, there are concerns about the security of those who use free wi-fi networks offered by coffee shops and other businesses.
The study, commissioned by card protection and insurance firm CPP, highlights a cavalier attitude to keeping data safe.
According to the findings, nearly a quarter of private wireless networks have no password attached, making them accessible to criminals.
CPP fraud expert Michael Lynch said: 'We urge all wi-fi users to remember that any information they volunteer through public networks can easily be visible to hackers."