Sierra Tel (ST) won’t forget April 10. On that date this year, the California-based telecommunications company may have been taken down by two warring families of botnets that attacked an Internet of Things (IoT) device in its network.

An IoT Device With a Checkered History

When its customers in Mariposa and Oakhurst, California, started to complain that they had lost all connectivity, the company determined that all these customers were using the same modem, the ZyXel HN51.

While ST diagnosed the cause of the problem rather quickly, Bleeping Computer reported that it took until April 22 for all the affected customers to obtain replacement devices. Frustrated customers quickly exhausted available supplies of the modem when the company offered them an opportunity to swap out old devices at its offices.

The ZyXel HN51 has a checkered history. This is the same modem that caused Deutsche Telekom to go offline for nearly a full day last year, according to Bleeping Computer. It took the German ISP about that long to regain control over its devices through a firmware update. A week later, some British ISPs experienced the same problem. At the time, the Mirai IoT botnet was thought to be the culprit.

The ZyXel modem uses the TR-069 control interface as a way for system administrators to assert hardware-level control on modems in a network. But that interface can be exploited, according to SANS, requiring strict filtering at the network or modem interface to prevent the exploits from occurring.

Vigilante Justice

It’s possible that a vigilante IoT construct could have caused this failure while trying to neutralize rogue IoT devices. One such construct, BrickerBot, is said to be able to wipe any onboard memory in a device and rewrite it with random garbage, Bleeping Computer reported. That would require device replacement, which is just what ST was forced to do.

A clear solution is nowhere in sight. Other unknown threat actors may be at work here as well, and this particular modem has proven to be exploitable. But a system operator such as ST must be aware of the actions that need to be implemented by a network to avoid bricked devices. No matter how or why they get bricked, they’ll almost always come with irate customers attached.