Researchers from the University of Valencia have uncovered a bug in several distributions of Linux that could allow anyone to bypass any kind of authentication during boot-up just by pressing backspace key 28 times. The issue is not in a kernel nor in an operating system itself, but rather the vulnerability resides in Grub2, the popular Grand Unified Bootloader. The vulnerability affects Linux systems from December 2009 to the present date, though older Linux systems may also be affected. The researchers have made an emergency patch to fix the vulnerability.
Source: https://thehackernews.com/2015/12/hack-linux-grub-password.html