The correct answer is to create a cross-account IAM role with an external ID and allow the auditor to assume the role using AWS STS. This is the most secure approach because the external ID acts as a secret password that the auditor must provide when assuming the role, preventing the confused deputy problem where a malicious third party could trick your role into granting unintended access. Combined with STS temporary credentials, this solution enforces time-limited, keyless access to the S3 bucket without ever exposing long-term access keys or static permissions. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of the cross-account role external ID S3 pattern, often appearing as a trap where candidates mistakenly choose presigned URLs or ACLs. The common trap is thinking presigned URLs work for bucket-wide audits—they only cover individual objects. Memory tip: think “External ID = secret handshake” for the confused deputy defense.
SCS-C02 Identity and Access Management Practice Question
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.
A company wants to allow a third-party auditor to read objects in an S3 bucket for a limited time. The auditor does not have an AWS account. What is the most secure way to grant this access?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a cross-account IAM role with an external ID and allow the auditor to assume the role using AWS STS.
Option C is correct because it uses cross-account roles with external ID and temporary credentials, which provide time-limited access without exposing long-term keys. Option A is wrong because sharing keys violates security best practices. Option B is wrong because ACLs do not support cross-account access without granting ListBucket permission. Option D is wrong because presigned URLs are typically used for individual objects, not a bucket-wide audit.
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.
✗
Use a bucket policy that grants read access to the auditor's AWS account (if they have one).
Why it's wrong here
The auditor does not have an AWS account, so this is not possible.
✗
Generate presigned URLs for all objects in the bucket and share them with the auditor.
Why it's wrong here
Presigned URLs are for individual objects, not bucket-wide access, and managing them at scale is impractical.
✓
Create a cross-account IAM role with an external ID and allow the auditor to assume the role using AWS STS.
Why this is correct
This provides time-limited, secure access without sharing permanent credentials.
Related concept
Standard ACLs match source addresses.
✗
Create an IAM user with read-only access and share the credentials with the auditor.
Why it's wrong here
Sharing IAM user credentials is insecure and against best practices.
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.
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 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
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 SCS-C02 ACL questions on filtering logic and placement.
Identity and Access Management — This question tests Identity and Access Management — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Create a cross-account IAM role with an external ID and allow the auditor to assume the role using AWS STS. — Option C is correct because it uses cross-account roles with external ID and temporary credentials, which provide time-limited access without exposing long-term keys. Option A is wrong because sharing keys violates security best practices. Option B is wrong because ACLs do not support cross-account access without granting ListBucket permission. Option D is wrong because presigned URLs are typically used for individual objects, not a bucket-wide audit.
What should I do if I get this SCS-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 SCS-C02 ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Question Discussion
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