Random lock up on web page, could String be doing it?

in my script I use the command:

String HTTP_req;

To catch data from the Ethernet client. Normal request seem to work fine but I am wondering if I may be running out of memory. When I load my program into the Arduino Uno, I am close to using all program space.

So the question!
Since this device is on the WWW and accessible to who knows what, could a random (hacker?) HTTP request overwhelm memory?

in my script I use the command:

String HTTP_req;

That isn't a command. It is a variable declaration.

I am wondering if I may be running out of memory.

Check for yourself.
http://playground.arduino.cc/Code/AvailableMemory

Since this device is on the WWW and accessible to who knows what, could a random (hacker?) HTTP request overwhelm memory?

Possibly.