- A
Create an IAM role in Account B that Account A can assume through STS, then grant the role S3 permissions.
Cross-account role assumption with AWS STS is the standard way to grant temporary access without sharing long-lived credentials. By placing the permissions on a role in Account B and controlling the trust policy there, the bucket-owning account keeps central control and can revoke access by changing the trust relationship or permissions. The Lambda execution role in Account A assumes the role when needed and receives short-lived credentials only.
- B
Create an IAM user in Account B and store its access keys in Lambda environment variables.
Why wrong: Static access keys are long-lived credentials and create unnecessary operational and security risk.
- C
Attach a security group to the Lambda function that allows outbound traffic to the bucket.
Why wrong: Security groups do not grant S3 permissions, and Lambda does not use security groups as an authorization mechanism for S3 access.
- D
Use AWS Organizations SCPs to grant the Lambda function permission to write to the bucket.
Why wrong: SCPs can only restrict permissions, not grant access to a specific workload or resource.
Quick Answer
The answer is to create an IAM role in Account B that Account A can assume through STS, then grant the role S3 permissions. This approach is correct because it leverages cross-account IAM roles with AWS Security Token Service (STS) to provide temporary credentials to the Lambda function, completely eliminating the need for long-lived access keys. The permissions are easily revocable from Account B by simply modifying or deleting the role, which meets the security requirement for immediate revocation. On the SAA-C03 exam, this scenario tests your understanding of cross-account Lambda access to S3 without long-lived keys, often appearing as a trap where candidates mistakenly choose resource-based policies or access keys. Remember that cross-account Lambda invocations require the role in the target account, not a policy on the Lambda itself. Memory tip: "Assume the role, not the key" — when you need cross-account access without permanent credentials, think STS and IAM roles.
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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 Lambda function in Account A must upload reports to an S3 bucket in Account B. Security does not want long-lived access keys anywhere, and the access should be easy to revoke from Account B. Which approach is best?
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.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Create an IAM role in Account B that Account A can assume through STS, then grant the role S3 permissions.
Option A is correct because it uses cross-account IAM roles with AWS Security Token Service (STS) to grant temporary credentials to the Lambda function. This avoids long-lived access keys, and the permissions can be revoked immediately by modifying or deleting the role in Account B, meeting the security requirements.
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.
- ✓
Create an IAM role in Account B that Account A can assume through STS, then grant the role S3 permissions.
Why this is correct
Cross-account role assumption with AWS STS is the standard way to grant temporary access without sharing long-lived credentials. By placing the permissions on a role in Account B and controlling the trust policy there, the bucket-owning account keeps central control and can revoke access by changing the trust relationship or permissions. The Lambda execution role in Account A assumes the role when needed and receives short-lived credentials only.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Create an IAM user in Account B and store its access keys in Lambda environment variables.
Why it's wrong here
Static access keys are long-lived credentials and create unnecessary operational and security risk.
- ✗
Attach a security group to the Lambda function that allows outbound traffic to the bucket.
Why it's wrong here
Security groups do not grant S3 permissions, and Lambda does not use security groups as an authorization mechanism for S3 access.
- ✗
Use AWS Organizations SCPs to grant the Lambda function permission to write to the bucket.
Why it's wrong here
SCPs can only restrict permissions, not grant access to a specific workload or resource.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse security groups (network-layer controls) with IAM policies (identity-based access), or mistakenly think SCPs can grant cross-account permissions when they only act as guardrails.
Detailed technical explanation
How to think about this question
When the Lambda function assumes the IAM role in Account B via STS, it receives temporary credentials (access key, secret key, session token) valid for up to 1 hour by default (configurable up to 12 hours). The role's trust policy must explicitly allow the Lambda execution role in Account A to assume it, and the S3 bucket policy in Account B must grant the role's ARN the necessary s3:PutObject permissions. This pattern is commonly used for cross-account data ingestion pipelines where audit trails and revocation are critical.
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.
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Secure Architectures — This question tests Design Secure Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create an IAM role in Account B that Account A can assume through STS, then grant the role S3 permissions. — Option A is correct because it uses cross-account IAM roles with AWS Security Token Service (STS) to grant temporary credentials to the Lambda function. This avoids long-lived access keys, and the permissions can be revoked immediately by modifying or deleting the role in Account B, meeting the security requirements.
What should I do if I get this SAA-C03 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". 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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Last reviewed: Jun 11, 2026
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