Standard accounts are able to get root access using sudo without password request

If you look at the very bottom of your sudoers file, it says "ALL ALL=(ALL) NOPASSWD:ALL". This is the syntax for passwordless execution of sudo. Make a backup of the file and then delete that line.

Andrew

Remove this line:

ALL ALL=(ALL) NOPASSWD:ALL

But this is REALLY strange to appear, so the software (basically a script I guess) you installed is probably mallicious. You might want to start a second question, paste the contents of that script there, and ask for advice... this is scary!