Signed System Volume

Sealed System Volume (SSV) is a security mitigation, introduced in macOS 11 Big Sur and then iOS/iPadOS 15. It is the reason that jailbreaks are moving towards the rootless architechture.

When a version above or including these versions is installed on device, the root hash image is also placed in the filesystem.

When the device is booted, iBoot verified that the contents of the System volume matched this root hash. If it doesn't, then the device will refuse to boot and will need to be restored.

It should be noted that certain directories, such as /var and /private/preboot, are designated to have differing files between devices and therefore do not have SSV applied to them.

On A11 and below, an SSV-bricked device can be fixed with a restore. However, currently, breaking the SSV seal on A12 and above leaves the device in a bricked state - as it causes restored_external to panic when trying to restore or update. Fortunately, this was recently made aware to Apple and should hopefully be fixed soon.

SSV does not apply to other disk partitions, hence why rootful palera1n can work by created a fakefs using other disk partitions.

To get around SSV, a BootROM or iBoot exploit is needed. However, there have been userland solutions demonstrated, such as utilising bind mounts.