Around the time of the snowden leaks we learned that the NSA was installing backdoors on Cisco firewalls bound for foreign countries.
https://en.wikipedia.org/wiki/Cisco_Systems#Firewall_backdoo...
"A document included in the trove of National Security Agency files released with Glenn Greenwald's book No Place to Hide details how the agency's Tailored Access Operations (TAO) unit and other NSA employees intercept servers, routers and other network gear being shipped to organizations targeted for surveillance and install covert firmware onto them before they are delivered. These Trojan horse systems were described by an NSA manager as being "some of the most productive operations in TAO because they pre-position access points into hard target networks around the world."