A recent report raised concerns about the security of certain devices manufactured by a company called DblTek. Trustwave SpiderLabs issued a report on a backdoor password scheme it found rampant in DblTek GoIP GSM gateways and other products that allow a remote user root-level Telnet access to the device. The attacker would then be able to execute arbitrary code and gain full control of the device.

A Widespread Occurrence

Hong Kong-based DblTek manufactures IP phones, SIM servers, various voice-over-IP (VoIP) equipment and cross-network gateways, noted Bleeping Computer. Gateways are often used by VoIP and telcom providers to link GSM and classic IP networks together. Trustwave confirmed that the GoIP 1, 4, 8, 16 and 32 models of the gateway all contain the potentially dangerous backdoor.

Other DblTek products were not vulnerability tested by the researchers, but they did not rule out that the problem could be larger than just the gateways.

The Hidden Account

Trustwave found that an account named “dbladm” was present on the gateway device firmware and was not controlled through the administration console. This hidden account can provide root-level access to the device, yet is not included in any documentation. Neither are the default “ctlcmd” and “limitsh” Telnet accounts.

This account also runs differently than the default accounts. Instead of using a traditional password, the hidden account uses a proprietary challenge-response authentication scheme for protection. This method of authentication gives the user a string. The user then performs various operations on that string to discover the password needed to continue.

This lends itself to an automated attack that is fairly simple to perform. An attacker could develop automated scripts to read the challenge and compute the response, thereby authenticating themselves on the device through the backdoor password.

Why Is the Backdoor Password Available?

Trustwave reported this situation to the vendor on Oct. 13, 2016. DblTek responded by issuing a patched version of the firmware on Dec. 22, 2016. But this patch does not eliminate the dbladm account from the gateway firmware; it just makes the authentication for it slightly more complex.

“It seems DblTek engineers did not understand that the issue is the presence of a flawed challenge response mechanism and not the difficulty of reverse engineering it,” stated Trustwave.

Why would the company keep such a backdoor? It may be a special account needed for testing devices, which has happened with other telcom products. Indeed, when activated, the account will send several UDP packets directed to the IP address 192.168.2.1 on port 11000/udp, Trustwave explained. This is consistent with the actions of a test account.

However, the DblTek response to not properly seal this particular vulnerability even when notified indicated that, for whatever reason, it wants the backdoor to exist — even if it slightly more obscured. Gateway users need to be aware of the potential vulnerability present in their equipment and deal with it accordingly.