Weak File Permissions

Example:

Readable/writeable /etc/shadow

Use JohnTheRipper to crack it ( Readable)

mkpasswd -m sha-512 PASSWORD

edit /etc/shadow and replace root password with yours ( Writeable )

Writeable /etc/passwd

openssl passwd PASSWORD

Edit passwd file and place generated password to root user's row replacing X

OR copy root user's row to the bottom of the file, change first instance to any name and place generated password hash replacing the X

TIP: Config files may contain sensitive information (passwords) to privesc as well.