Eircom says it will be getting in touch with up to 250,000 of its customers following concerns about security on its wireless modems.
The security flaw means that hackers could access wireless connections without the knowledge of the account holder.
The issue affects the Netopia 3300 and 2247 series routers.
Wireless broadband systems have always been vulnerable to abuse because other people besides the subscriber can use the Internet connection.
An Eircom spokesman said it would take someone with technical expertise to be able to 'piggy back' on someone else's broadband.
However he said that if it were to happen the account holder could be held responsible for the websites visited by a hacker.
He advised customers to change the four-digit security personal identification number provided with their wireless modems in order to provide a basic level of security.
But he pointed out that no wireless system offered complete protection from abuse.
Instructions on how to improve security are available on Eircom's broadband website.
