Ask HN: Unknown IPs visiting my mock web app. Should I be worried?

I am renting a VM at Digital Ocean to make a small web app. Nothing is ready yet and I have only given the link to some friends, and today while checking my log I saw something like:

    INFO:werkzeug:186.237.38.73 - - [14/May/2014 01:14:17] "GET /webman/info.cgi?host=HTTP/1.0" 404 -
    INFO:werkzeug:125.108.128.98 - - [14/May/2014 08:48:10] "GET /webman/info.cgi?host= HTTP/1.0" 404 -
    INFO:werkzeug:198.20.70.114 - - [14/May/2014 09:42:07] "GET / HTTP/1.1" 200 -
    INFO:werkzeug:198.20.70.114 - - [14/May/2014 09:42:07] "GET /robots.txt HTTP/1.1" 404 -
    

Those IPs seem to be from Brazil, which is not from where I or the friends I gave my link are from. I suspect they are people trying to see if my VM is hackable, but I don't have the technical knowledge to see exactly what they want and how I can prevent them to take control of my VM. For example, do I need to set up robots.txt or webman/info.cgi?host= ?