While it's true that you might be able to mount the System volume as RW you would still need to bypass SIP to compromise the machine.

This volume is the one that gets encrypted if you enable FileVault.Ī malicious actor that wanted to tamper with the system would have to be able to either circumvent SIP and gain write access to the System image or unlock the System - Data volume by using a user password or recovery key. The System - Data volume which contains both user and system mutable data is mapped on top of it by the use of firmlinks at boot time. This volume is not encrypted (as in apfs encrypted with a user key, as we saw everything is encrypted by the T2 chip) but it can be only written by apple installers. In Catalina the System volume is read only (additionally it's protected under SIP) and gets mounted at / during boot. You raise a very valid point and the answer is far from obvious. If you enable filevault and use target disk mode you need to supply the password of a user of the computer to unlock the disk.

If you enable disk encryption and use target disk mode you need to supply the disk password to unlock the disk. If someone connected your computer in target disk mode without Filevault the disk would be automatically unlocked by using the key stored in the secure enclave of the T2 chip. With Filevault the disk password is tied to user account passwords so when the system boots it doesn't have to ask you for the disk password to unlock it and you can have multiple users able to unlock the disk without the need of them sharing a single disk password. With basic APFS encryption you supply a disk password and that's required to unlock the disk. The only difference with APFS encryption is how the key is handled. You should just enable Filevault and you will be alright. On T2 equipped drives APFS uses T2 hardware acceleration to improve the speed of encryption, so again it is the same thing. FileVault uses APFS encryption, they are essentially the same thing.

Apfs encrypted