As involved as that process was, getting unauthorized code covertly installed into an official operating system and keeping it there for years would appear to be an even more complicated—and brazen—undertaking. This 2013 article published by Der Spiegel reported that an NSA operation known as FEEDTHROUGH worked against Juniper firewalls and gave the agency persistent backdoor access.

Source: “Unauthorized code” in Juniper firewalls decrypts encrypted VPN traffic | Ars Technica