According to an alert issued Tuesday by the CERT Coordination Center (CERT/CC) at Carnegie Mellon University, the affected device models are: ASUS DSL-N12E, DIGICOM DG-5524T, Observa Telecom RTA01N, Philippine Long Distance Telephone (PLDT) SpeedSurf 504AN and ZTE ZXV10 W300.
All of the devices have an admin password in the form "XXXXairocon" where XXXX are the last four characters of the device's physical MAC address, CERT/CC said.
Using a portion of the MAC address, which is unique for every device, does not make it harder for an attacker to find out the password, because a device's full MAC address can be obtained by sending a public query over the Simple Network Management Protocol (SNMP).
For most of the routers, the username corresponding to the hard-coded password is admin, while for the PLDT SpeedSurf 504AN it's adminpldt. A remote attacker may utilize these credentials to gain administrator access to the affected devices over their telnet service, CERT/CC said.
The vulnerability is not new and was independently reported by separate researchers in 2014 for the ZTE ZXV10 W300 and in May for the Observa Telecom RTA01N. However, it hasn't been known until now that other devices were also affected.
Since the hard-coded password has the same format for all the mentioned devices, it's likely that their firmware was developed by the same company. It's not unusual in the embedded device world for one vendor to contract other companies to develop firmware development or even to manufacture their hardware.
Earlier this year a security researcher found identical vulnerabilities in a large number of DSL router models from different manufacturers that were distributed by ISPs from around the world to their customers. Despite the different branding on the devices, all of them were running firmware developed by a single Chinese company.