Ubuntu Security Notice 868-1 – It was discovered that GRUB 2 did not properly validate passwords. An attacker with physical access could conduct a brute force attack and bypass authentication by submitting a 1 character password.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/83620/USN-868-1.txt
Source: https://packetstormsecurity.com/files/83620/Ubuntu-Security-Notice-868-1.html