Is this a security issue ?
Yes and not, for Gnu/Linux systems if you have physical access to a computer you are a super-admin, and so you can use this method, or start with a live distribution to recover all the credentials of the system.
If you are worried about this you should probably set various layer of protection such as Password protected bios and grub and encrypted disk information (at least /home),


I will show some tip to get into the Linux System if you forgot your password..