There are several articles that says that vendors plant backdoors on network devices intentionally, such as this one.
They found backdoors that were made by the vendors. Also, I read somewhere else that major network device companies put backdoors on devices to give support staff emergency access to devices. If they find a security vulnerability they want to be able to patch it easily without having to make the user download and install a fix.
Note that these kinds of backdoors that are my concern are not the type that Ed Snowden called "government backdoors".
Are these backdoors real? Are they necessary? Is it also a potential danger?