The answer is that a higher priority allow rule is overriding the block rule. AWS WAF evaluates rules in ascending order of priority, where a lower numeric value means higher priority; if an allow rule with priority 1 permits all traffic, a block rule with priority 2 will never be reached because the request is already allowed. This scenario tests your understanding of WAF rule evaluation order, a common trap on the AWS Certified Security Specialty SCS-C02 exam where candidates overlook that allow rules can preempt block rules even when the block condition matches. The search intent “WAF rule not blocking requests due to higher priority allow rule” directly reflects this misconfiguration—always check rule priorities before debugging conditions. A useful memory tip: in WAF, priority is like a bouncer’s list—if the first bouncer says “let everyone in,” the second bouncer never gets to check IDs.
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company has a web application running on EC2 instances behind an Application Load Balancer (ALB). The application uses a custom header X-Auth-Token to authenticate requests. The security team wants to use AWS WAF to block requests that do not contain this header or contain an invalid token. The WAF is associated with the ALB. The team creates a rule with a match condition that checks for the presence of the X-Auth-Token header and a regex pattern for the token value. However, the rule is not blocking any requests. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
There is an allow rule with a higher priority that allows all requests before the block rule is evaluated.
Option D is correct. AWS WAF evaluates rules in order, and if a rule with a higher priority allows the request, it will not be blocked by a lower priority rule. The team may have an allow rule with higher priority that allows all requests. Option A is incorrect because WAF can inspect headers. Option B is incorrect because WAF can be used with ALB. Option C is incorrect because WAF can handle regex.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
AWS WAF is not supported for Application Load Balancers; it only supports CloudFront.
Why it's wrong here
WAF supports ALB.
✗
AWS WAF cannot inspect custom headers; it can only inspect standard HTTP headers.
Why it's wrong here
WAF can inspect custom headers.
✗
The regex pattern for the token is too complex for AWS WAF to process.
Why it's wrong here
WAF supports regex patterns.
✓
There is an allow rule with a higher priority that allows all requests before the block rule is evaluated.
Why this is correct
Rules are evaluated in priority order; a higher priority allow rule would override the block rule.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SCS-C02 NAT questions on configuration and troubleshooting.
Infrastructure Security — This question tests Infrastructure Security — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: There is an allow rule with a higher priority that allows all requests before the block rule is evaluated. — Option D is correct. AWS WAF evaluates rules in order, and if a rule with a higher priority allows the request, it will not be blocked by a lower priority rule. The team may have an allow rule with higher priority that allows all requests. Option A is incorrect because WAF can inspect headers. Option B is incorrect because WAF can be used with ALB. Option C is incorrect because WAF can handle regex.
What should I do if I get this SCS-C02 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related SCS-C02 NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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