Network provider EE will push out an emergency upgrade to its broadband customers after a security flaw was discovered by a UK researcher.
Scott Helme said the vulnerability made "remote access" to EE's routers possible.
The problem affects customers who have either the Brightbox 1 or 2 router in their homes.
EE described the threat as "moderate", but plans to send out an automatic upgrade before the end of this month.
Any broadband customer who has signed up to EE since early 2012 is affected, as are earlier customers who upgraded
It has not specified how many of its customers will need the upgrade, but it is expected to be in the region of 350,000.
In a statement, EE said: "We treat all security matters seriously, and while no personal data will be compromised by the device itself, we would like to reassure customers that we are working on a service update which we plan to issue shortly, and which will remotely and automatically update customers' Brightboxes with enhanced security protection."