The Security Alert for Allwinner sun8i thread on Armbian forums is interesting.
found via The Register article This is what a root debug backdoor in a Linux kernel looks like. Interestingly enough this backdoor was created by Allwiner itself, and might exist on other Allwinner devices.
Look for the file
/proc/sunxi_debug/sunxi_debug, if it exists, try to write "rootmydevice" to it. Simple enough.
(found via LXer)