A number of Broadcom's and another unnamed vendor's Wi-Fi router chipsets are vulnerable to being controlled by hackers.
Research by Swiss security firm Oxcite said attackers could access passwords inside the firms' routers within seconds by taking advantage of randomisation techniques.
This is because the firms had poorly implemented versions of the Wi-Fi Protected Setup (WPS), which allowed hackers to calculate the correct key offline.
Dominque Bongard, reverse engineer and founder of Oxcite, said Broadcam products had poor randomisation, while the second vendor used a non-random seed value or nonce.
He added that the new attack only requires a single guess and a series of offline calculations, according to Ars Technica.
"It takes one second," he said. "It's nothing. Bang. Done."
He said the flaw could affect many providers because they use the same reference software to build their custom router gear.
"A vendor implementation that improperly generates random numbers is more susceptible to attack, and it appears as though this is the case with at least two devices. It is likely that the issue lies in the specific vendor implementations rather than the technology itself," Wi-Fi Alliance's Kevin Robinson, director of programme marketing, said in a statement.
"As the published research does not identify specific products, we do not know whether any Wi-Fi certified devices are affected, and we are unable to confirm the findings."
The research, which was presented at the PasswordsCon conference in Las Vegas last month, advised users to disable their WPS until the flaws are fixed.