Quick Answer
The answer is that an explicit deny in the S3 bucket policy overrides the IAM allow. This occurs because AWS authorization logic follows a strict hierarchy: explicit denies always take precedence over any allow, regardless of whether the allow comes from an IAM user policy or a resource-based bucket policy. In this scenario, the IAM policy grants s3:ListBucket, but the bucket policy contains an explicit deny for the same action, which is why the user receives an AccessDenied error. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of the "deny override" principle, often appearing as a trick where a seemingly sufficient IAM policy is silently blocked by a bucket policy. A common trap is assuming IAM allows are sufficient without checking resource-based policies. Memory tip: think of explicit deny as a "veto" — no matter how many "yes" votes (allows) you have, one "no" (explicit deny) kills the action.
This SCS-C02 practice question tests your understanding of identity and access management. 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.
An IAM user receives an 'AccessDenied' error when trying to list objects in an S3 bucket. The user has the following policy attached: {"Version":"2012-10-17","Statement":[{"Effect":"Allow","Action":"s3:ListBucket","Resource":"arn:aws:s3:::example-bucket"}]}. What is the most likely reason?
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.
The policy is missing a condition
Why wrong: Conditions are not required.
The bucket policy explicitly denies the action
An explicit deny in the bucket policy overrides the user policy allow.
The policy does not include s3:GetObject
Why wrong: ListBucket is sufficient to list objects; GetObject is for reading objects.
The policy has a syntax error
Why wrong: The policy is valid JSON.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The bucket policy explicitly denies the action
The IAM policy grants the s3:ListBucket action on the bucket, which should allow listing objects. However, an explicit deny in a bucket policy overrides any allow, including those from IAM policies. Since the user receives an 'AccessDenied' error, the most likely cause is that the bucket policy explicitly denies the s3:ListBucket action for this user, as explicit denies take precedence over all allows.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The policy is missing a condition
Why it's wrong here
Conditions are not required.
The bucket policy explicitly denies the action
Why this is correct
An explicit deny in the bucket policy overrides the user policy allow.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The policy does not include s3:GetObject
Why it's wrong here
ListBucket is sufficient to list objects; GetObject is for reading objects.
The policy has a syntax error
Why it's wrong here
The policy is valid JSON.
Common exam traps
The trap here is that candidates often assume an IAM policy alone is sufficient and forget that bucket policies can explicitly deny actions, overriding IAM allows, leading them to incorrectly choose options like missing permissions or syntax errors.
Detailed technical explanation
In AWS IAM, the evaluation logic follows an 'explicit deny overrides allow' rule. When both an IAM policy and a bucket policy apply, the bucket policy can include a Deny statement that explicitly blocks the s3:ListBucket action for a specific principal, which overrides any Allow from the IAM policy. This is a common security pattern to enforce cross-account restrictions or prevent unauthorized listing even if an IAM policy grants it.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
Identity and Access Management — This question tests Identity and Access Management — Read the scenario before looking for a memorised answer..
The correct answer is: The bucket policy explicitly denies the action — The IAM policy grants the s3:ListBucket action on the bucket, which should allow listing objects. However, an explicit deny in a bucket policy overrides any allow, including those from IAM policies. Since the user receives an 'AccessDenied' error, the most likely cause is that the bucket policy explicitly denies the s3:ListBucket action for this user, as explicit denies take precedence over all allows.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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Variation 1. A developer is trying to use the AWS CLI to list objects in an S3 bucket but receives an AccessDenied error. The developer has an IAM user with a policy that allows s3:ListBucket on the bucket. What could be causing the error?
easyWhy B: Option A is correct because even if the IAM policy allows the action, the bucket policy may explicitly deny access. Option B is wrong because MFA is not required unless a condition exists. Option C is wrong because the developer can have multiple policies that combine. Option D is wrong because the bucket exists if the error is AccessDenied.
Last reviewed: Jun 11, 2026
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