The answer is a cached deny policy from a previous invalid token. When API Gateway Lambda authorizer caching is enabled, the authorizer result—including a deny IAM policy—is stored for the Time to Live (TTL) duration. If a request with a valid token arrives but the cache holds a deny for that same token (from an earlier failed attempt), API Gateway returns a 401 Unauthorized error without re-invoking the Lambda function, which explains why logs show the function is invoked only for the first invalid request. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of API Gateway’s caching behavior and the distinction between token validation and policy caching—a common trap is assuming the token itself is invalid when the issue is stale cache. Remember the mnemonic: “Cache the policy, not just the token—deny lingers until TTL is broken.”
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 uses Amazon API Gateway with a Lambda authorizer to control access to its APIs. Recently, some requests are returning 401 Unauthorized errors even though the tokens are valid. The Lambda authorizer logs show that the function is invoked and returns an IAM policy. 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
✓
The authorizer result is cached, and a previous invalid token caused a deny policy to be cached.
Option B is correct because API Gateway caches authorizer responses; if the cache is enabled and the token is cached with a deny, subsequent requests with the same token will fail. Option A is wrong because the authorizer is being invoked. Option C is wrong because token expiration would cause a different error. Option D is wrong because the Lambda function returns a policy.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The Lambda function is returning a malformed IAM policy.
Why it's wrong here
Malformed policies cause 500 errors, not 401.
✗
The token has expired.
Why it's wrong here
Token expiration would be a different error, but the authorizer is invoked.
✗
The Lambda authorizer is not being invoked due to throttling.
Why it's wrong here
Logs show invocation, so throttling is not the issue.
✓
The authorizer result is cached, and a previous invalid token caused a deny policy to be cached.
Why this is correct
Cached deny responses cause subsequent valid tokens to be rejected.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Standard ACLs match source addresses.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Trap categories for this question
Command / output trap
Logs show invocation, so throttling is not the issue.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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 ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SAP-C02 ACL questions on filtering logic and placement.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The authorizer result is cached, and a previous invalid token caused a deny policy to be cached. — Option B is correct because API Gateway caches authorizer responses; if the cache is enabled and the token is cached with a deny, subsequent requests with the same token will fail. Option A is wrong because the authorizer is being invoked. Option C is wrong because token expiration would cause a different error. Option D is wrong because the Lambda function returns a policy.
What should I do if I get this SAP-C02 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related SAP-C02 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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