A security flaw on Virgin Media's Super Hub 2 routers is also present on devices issued by other providers, experts have warned.
Last week, consumer group Which? said the routers could be vulnerable to hackers if they are left with the default password that comes with them.
Virgin Media has therefore contacted 800,000 of its customers who use Super Hub 2 routers and encouraged them to change their passwords.
However, security firm Pen Test Partners believes "it's a bit unfair that Virgin Media has been singled out here", as the same mistake has also been made by other major broadband providers.
Ken Munro, spokesperson for the company, told BBC News: "This problem has been known about for years, yet still ISPs issue routers with weak passwords and consumers don't know that they should change them."
"Because the default Wi-Fi password formats are known, it's not difficult to crack them."
Mr Munro added that "very few" people change their Wi-Fi password from the one written on the router.
Read uSwitch's guide to broadband safety for more information on how to keep your data safe and secure.