You find the Security tab under Server -> Server Settings.
On the Security tab you can specify which computer has the right to execute extractions via HTTP.
Allow HTTP access to everybody
There are no limitations. Every machine is allowed to execute extractions.
Restrict HTTP access to specific machines
Only defined computers are allowed to execute extractions.
To define a restriction select Restrict HTTP access to specific machines and click the Add button.
Now you can set the restrictions for single computers (IP-address) of for computers from a specified subnet or a special name pattern. To define a name pattern you can use regular expressions.