The Browser Security setting option will not appear on applications that were created prior to 4.2 upgrade.

It will, however, appear on new applications created.

To enable the Browser Security setting on an existing application, navigate to: 

Application > Edit Application Properties (Edit Application Definition)

Next, enable compatibility mode for 4.2

You will now find the Browser Security setting under:

Shared Components > Security Attributes

If you have any questions, please raise a support ticket.