I just looked this up and to save others the time, this was an integer underflow bug in the Grub2 bootloader's password protection[0], not Linux itself. Cool example, though.
[0] http://hmarco.org/bugs/CVE-2015-8370-Grub2-authentication-by...
[0] http://hmarco.org/bugs/CVE-2015-8370-Grub2-authentication-by...
The OP specifically said the login prompt was defeated by backspacing alone.
https://groups.google.com/forum/#!msg/muc.lists.bugtraq/5zYU...