The VPNFilter attack in 2018 was a sophisticated malware threat as documented by Cisco Talos below.  Thankfully, it was shutdown by FBI before major impacts were experienced:

https://blog.talosintelligence.com/2019/05/one-year-later-vpnfilter-catastrophe.html#more

Cisco Talos first disclosed the existence of VPNFilter on May 23, 2018. The malware made headlines across the globe, as it was a sophisticated piece of malware developed by a nation state, infecting half a million devices, and poised to cause havoc. Yet the attack was averted. The attacker’s command and control (C2) infrastructure was seized by the FBI, preventing the attacker from broadcasting orders to compromised devices. The attacker lost control of the infected systems, and potential catastrophe was prevented.

This was a wakeup call that alerted the cybersecurity community to a new kind of state-sponsored threat — a vast network of compromised devices across the globe that could stow away secrets, hide the origins of attacks and shut down networks.

This is the story of VPNFilter, and the catastrophe that was averted. Network infrastructure is a tempting and useful target to attackers. Like any computing system, network devices such as routers and switches may contain vulnerabilities or misconfigurations that allow attackers to compromise the device. Once compromised, the device can be used as a point of incursion to search out and attack additional further systems, or the functionality of the device can be changed to the attacker’s will, and network traffic intercepted, modified or rerouted. Unlike many other computing systems, routers and switches are unlikely to be running anti-virus software, or be under active supervision by eagle-eyed administrators who may notice unusual activity.