The vendor wasn't named by the researchers, David Byrne and Charles Henderson, but they did disclose the password: 166816.
A Google search reveals that's the default password for several models of credit card terminal sold by Verifone, a Silicon Valley-based vendor that says it connects 27 million payment devices and has operations in 150 countries.
Verifone didn't immediately comment on the claim.
The researchers said that the password remains in use on nine out of ten terminals they see from the vendor, in part because customers mistakenly assume it is unique to them.
The RSA Conference is a major annual event for the security industry and the comments of the two researchers were reported by The Register and SC Magazine.
Such conferences are often used to highlight poor security practices or disclose how hackers are targeting systems. The hope is that by making such information more widely known, companies and users will be more alert to cybersecurity and change bad habits.
Cybercriminals have been increasingly targeting vulnerabilities in point-of-sale terminals.
The largest and best known of these was the hack of Target, which affected up to 70 million customers. A breach at Home Depot compromised the payment cards of up to 56 million customers, while systems have also been hacked at Neiman Marcus, White Lodging, Michaels and The UPS Store.
In part because of these breaches, the payments industry in the U.S. is moving to chip-based cards, but researchers note the security on the cards isn't bullet-proof and in part depends on the payment terminals and other systems at retailers.
Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com