Recent App Test

During a recent Application pen test I came across a url of the form:
 
http://www.vulnsite.com/cgi-bin/vulnscript.jsp?url=www.website.com&id=12345
 
I changed the url parameter to something like url=www.google.com and google 
appeared in my browser. Next, i changed the url to url=www.whatismyip.com, 
hoping that the ip address of the webserver would be displayed, however, only 
my ip address was displayed.

This means that my browser is loading the url parameter as opposed to the 
webserver script fethching the url and then displaying it for me in my browser 
right? Is this a security issue?


Assuming that it was the actual webserver script fetching the url parameter and 
then displaying it for me, I've come up with a few vulnerabilities (listed 
below) and was hoping that people might like to share some of their ideas.
   
1) Can use vulnsite as a proxy (& hack other sites)
2) Can port scan using the vuln site by changing url=www.website.com to 
url=www.sitetoscan.com:port
3) Can connect to & port scan machines behind the firewall.
 

Thanks in adance,
Sol