Replicant developer Paul Kocialkowski explains further in the blog post: (emphasis mine)
Today's phones come with two separate processors: one is a general-purpose applications processor that runs the main operating system, e.g. Android; the other, known as the modem, baseband, or radio, is in charge of communications with the mobile telephony network. This processor always runs a proprietary operating system, and these systems are known to have backdoors that make it possible to remotely convert the modem into a remote spying device. The spying can involve activating the device's microphone, but it could also use the precise GPS location of the device and access the camera, as well as the user data stored on the phone. Moreover, modems are connected most of the time to the operator's network, making the backdoors nearly always accessible.
Provided that the modem runs proprietary software and can be remotely controlled, that backdoor provides remote access to the phone's data, even in the case where the modem is isolated and cannot access the storage directly. This is yet another example of what unacceptable behavior proprietary software permits! Our free replacement for that non-free program does not implement this backdoor. If the modem asks to read or write files, Replicant does not cooperate with it.
The blog post contains pointers to more information including a technical description of the back-door found in Samsung Galaxy devices and a list of known affected Samsung Galaxy devices. The FSF lists more ways proprietary software is often malware."
Link to Original Source