An engineer has configured a Web Security rule that restricts access to certain web applications for a specific user group. During testing, the rule does not take effect as expected, and the users can still access blocked web applications. What is a reason for this issue?

Palo Alto Networks SSE (Prisma Access) evaluates Web Security rules in a top-down order. The first rule in the list that matches the traffic's attributes (source, destination, user, application, etc.) is applied, and all subsequent rules are ignored for that session. If a more general "allow" rule is positioned higher in the rule hierarchy than a more specific "deny" rule, the traffic will match the "allow" rule first. Consequently, the intended "deny" rule, being at a lower level, will never be evaluated for that traffic. This common configuration issue is known as rule shadowing.

A. Threat management settings (e.g., antivirus, anti-spyware profiles) are applied to traffic that has already been allowed by a security rule; they do not influence the initial allow or deny decision.
B. While an incorrect scope could be an issue, rule ordering is a more fundamental reason for a correctly scoped rule to fail, as a preceding rule can override it for all users.
C. This statement is logically incorrect. Rules at a higher level in the hierarchy have higher priority and are evaluated first, not the other way around.

1. Palo Alto Networks Prisma Access Administrator's Guide: In the section on Security Policy, it states, "Prisma Access evaluates security policy rules from top to bottom. The first rule that matches the traffic is applied, and no other rules are evaluated." This principle directly supports the concept that a higher-level rule will take precedence. (Reference: Security Policy > Security Policy Rules section).

2. Palo Alto Networks Prisma Access Administrator's Guide: When troubleshooting policies, the documentation highlights rule shadowing as a primary cause of unexpected behavior. "If a security policy rule is not matching traffic as you expect, it could be because a rule above it in the rulebase is matching the traffic first (this is known as rule shadowing)." This directly explains the scenario in the question. (Reference: Monitor > Troubleshoot Policies section).