The temptation to put a backdoor into a product is almost overwhelming. It’s just so dang convenient. You can go into any office, any lab, any customer site and get your work done. No hassles with getting passwords or clearances. You can just solve problems. You can log into any machine and look at logs, probe the box, issue commands, and debug any problem. This is very attractive to programmers.
I’ve been involved in several command line interfaces to embedded products and though the temptation to put in a backdoor has been great, I never did it, but I understand those who have.
There’s another source of backdoors: infiltration by an attacker.
We’ve seen a number of backdoors hidden in code bases you would not expect. Juniper Networks found two backdoors in its firewalls. Here’s Some Analysis of the Backdoored Backdoor. Here’s more information to reaffirm your lack of faith in humanity: NSA Helped British Spies Find Security Holes In Juniper Firewalls. And here are a A Few Thoughts on Cryptographic Engineering.
Juniper is not alone. Here’s a backdoor in AMX AV equipment. A Secret SSH backdoor in Fortinet hardware found in more products. There were Backdoors Found in Barracuda Networks Gear. And The 12 biggest, baddest, boldest software backdoors of all time. Who knows how many backdoors are embedded in chips? Security backdoor found in China-made US military chip. And so on.
By now we can pretty much assume backdoors are the rule, not the exception.