The answer is to use Cognito identity credentials with an IAM policy scoped to the user’s prefix using policy variables. This approach enforces least privilege S3 upload with Cognito identity pool by issuing temporary AWS credentials that are dynamically restricted to a specific S3 prefix, such as `uploads/${cognito-identity.amazonaws.com:sub}/`, ensuring each authenticated user can only write to their own folder. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of how policy variables like `${cognito-identity.amazonaws.com:sub}` work with Cognito identity pools to achieve fine-grained access control without hardcoding user IDs. A common trap is choosing static IAM roles or pre-signed URLs, which either grant overly broad access or require server-side logic. Remember the mnemonic: “Sub in the bucket” — the `sub` claim scopes the upload to the user’s personal prefix.
DVA-C02 Security Practice Question
This DVA-C02 practice question tests your understanding of security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 mobile application must let authenticated users upload only to their own S3 prefix. Which approach best follows least privilege?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Cognito identity credentials with an IAM policy scoped to the user's prefix using policy variables
Option A is correct because it uses Amazon Cognito identity pools to issue temporary AWS credentials scoped to a specific S3 prefix via IAM policy variables (e.g., `${cognito-identity.amazonaws.com:sub}`). This ensures each authenticated user can only upload to their own prefix (e.g., `uploads/${user_id}/`), adhering to the principle of least privilege by granting no more access than necessary.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 Cognito identity credentials with an IAM policy scoped to the user's prefix using policy variables
Why this is correct
Correct for the stated requirement.
Clue confirmation
The clue words "best", "least" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a single hardcoded access key in the app
Why it's wrong here
This does not meet the stated requirement as directly as the correct option.
✗
Make the bucket public and validate names in the client
Why it's wrong here
This does not meet the stated requirement as directly as the correct option.
✗
Give every user AmazonS3FullAccess
Why it's wrong here
This does not meet the stated requirement as directly as the correct option.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates might choose Option B (hardcoded key) thinking it's simpler, missing that it exposes a static credential that can be compromised, or Option C (public bucket) assuming client-side validation is sufficient, when in fact AWS requires server-side enforcement for security.
Detailed technical explanation
How to think about this question
Under the hood, Cognito identity pools use AWS Security Token Service (STS) to vend temporary credentials that include a role with a trust policy. The IAM policy uses the `${cognito-identity.amazonaws.com:sub}` variable, which resolves to the unique identity ID (e.g., `us-east-1:12345678-...`), ensuring each user's uploads are isolated to their own prefix. A real-world scenario is a photo-sharing app where users must only see their own images; without this scoping, a user could overwrite another's files.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Cognito identity credentials with an IAM policy scoped to the user's prefix using policy variables — Option A is correct because it uses Amazon Cognito identity pools to issue temporary AWS credentials scoped to a specific S3 prefix via IAM policy variables (e.g., `${cognito-identity.amazonaws.com:sub}`). This ensures each authenticated user can only upload to their own prefix (e.g., `uploads/${user_id}/`), adhering to the principle of least privilege by granting no more access than necessary.
What should I do if I get this DVA-C02 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best", "least". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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