About half of the internet connections in St Peter Port, Guernsey, are unprotected and vulnerable to abuse, an ethical hacker has said.
Matthew Parker, who investigates online security weaknesses for Ernst and Young, said 50% lacked passwords.
He said: "If you're not protecting yourself anyone could be using it and it could be used for illegal purposes."
Mr Parker said the hacking software was readily available over the internet so the threat was very real.
Matthew Morel, Channel Islands Airtel-Vodafone marketing manager, said: "Security protocols, like pin code access and web encryption passwords, before they actually start using the device, stop fraudulent use."
Tim Ringsdore, managing director of Wave Telecom, said computer users should also take "advice from friends and family who use sites to make sure that they are legitimate sites", to restrict spamming.