Root has no password. And when developer mode is on, telnet is running. So, to get basic shell access, all one needs to do is enable developer mode on the Kobo and enable Wifi. Login as root with no password.
(At least, this is true for 4.x firmwares I believe)
If you want something more sophisticated than telnet, NiLuJe's Kobo Stuff package includes dropbear to add SSH access.
One assumes the serial console will be similarly unprotected.
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