SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
In production, the goals are different. A secure backdoor is desired, but it must also be reliable. Firmware bugs which prevent backdoor access are less of a concern. All of the firmware, including the backdoor should be well tested before release to production.
The best firmware backdoor is a communications channel based backdoor with a long password, as described above. It could even be a sequence of multiple commands, if desired.