To enforce IDP policies on HTTP traffic on an SRX Series device, the following actions must be performed:

Choose an attacks type in the predefined-attacks-group HTTP-All: This allows the SRX Series device to match on specific types of attacks that can occur within HTTP traffic. For example, it can match on SQL injection or cross-site scripting (XSS) attacks.

Match on application junos-http: This allows the SRX Series device to match on HTTP traffic specifically, as opposed to other types of traffic. It is necessary to properly identify the traffic that needs to be protected.

Disabling screen options on the Untrust zone and specifying an action of None are not necessary to enforce IDP policies on HTTP traffic. The first one is a feature used to prevent certain types of attacks, the second one is used to take no action in case of a match.