- A
Allow the Account B root principal to assume the role if the caller knows the role ARN.
Why wrong: This would work technically, but it is far too broad and does not protect against confused deputy abuse. Any principal in Account B that can reach the role could potentially use it, which violates the customer’s security requirement.
- B
Allow only the vendor’s specific IAM principal to assume the role and require a unique sts:ExternalId condition.
This is the standard confused deputy protection pattern for third-party cross-account access. The trust policy limits who can call AssumeRole, and the sts:ExternalId condition lets the customer require a customer-specific value that the vendor must supply. That prevents another customer or a malicious party from reusing the same role ARN successfully.
- C
Attach a permissions boundary to the role so that the vendor cannot exceed the approved permissions.
Why wrong: A permissions boundary limits what permissions the role can have after it is assumed, but it does not stop an unintended principal from assuming the role. It addresses privilege scope, not confused deputy risk.
- D
Require MFA for the role assumption because it ensures only the vendor’s production automation can use the role.
Why wrong: MFA is not a practical control for non-human automation and does not solve cross-account confused deputy risk. The vendor’s automation should use a trust policy with a specific principal and an ExternalId instead.
Quick Answer
The answer is to allow only the vendor’s specific IAM principal to assume the role and require a unique sts:ExternalId condition. This design directly addresses the confused deputy prevention sts:ExternalId scenario because the external ID acts as a secret token that the customer controls, ensuring the vendor’s automation can only assume the role when acting on behalf of that specific customer—even if multiple customers use the same vendor software. On the SAA-C03 exam, this concept tests your understanding of cross-account trust policies and the confused deputy problem, often appearing in scenarios where a third-party service needs temporary access to your resources. A common trap is to think that restricting the source account or principal alone is sufficient, but without the external ID, a compromised vendor account could be tricked into assuming roles in other customer accounts. Remember the mnemonic: “External ID, external threat—without it, your trust policy is incomplete.”
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 SaaS vendor’s automation account in Account B needs to assume a role in a customer account in Account A to read a specific S3 bucket and publish a deployment status file. The customer is worried about confused deputy attacks because multiple customers use the same vendor software. Which trust-policy design best meets the requirement?
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
Allow only the vendor’s specific IAM principal to assume the role and require a unique sts:ExternalId condition.
Option B is correct because the `sts:ExternalId` condition is specifically designed to prevent the confused deputy problem in cross-account role assumptions. By requiring a unique external ID that only the customer knows, the customer ensures that the vendor's automation can only assume the role when acting on behalf of that specific customer, even if multiple customers use the same vendor software.
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.
- ✗
Allow the Account B root principal to assume the role if the caller knows the role ARN.
Why it's wrong here
This would work technically, but it is far too broad and does not protect against confused deputy abuse. Any principal in Account B that can reach the role could potentially use it, which violates the customer’s security requirement.
- ✓
Allow only the vendor’s specific IAM principal to assume the role and require a unique sts:ExternalId condition.
Why this is correct
This is the standard confused deputy protection pattern for third-party cross-account access. The trust policy limits who can call AssumeRole, and the sts:ExternalId condition lets the customer require a customer-specific value that the vendor must supply. That prevents another customer or a malicious party from reusing the same role ARN successfully.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Attach a permissions boundary to the role so that the vendor cannot exceed the approved permissions.
Why it's wrong here
A permissions boundary limits what permissions the role can have after it is assumed, but it does not stop an unintended principal from assuming the role. It addresses privilege scope, not confused deputy risk.
- ✗
Require MFA for the role assumption because it ensures only the vendor’s production automation can use the role.
Why it's wrong here
MFA is not a practical control for non-human automation and does not solve cross-account confused deputy risk. The vendor’s automation should use a trust policy with a specific principal and an ExternalId instead.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse MFA or permissions boundaries as solutions for the confused deputy problem, when in fact only the `sts:ExternalId` condition directly mitigates this specific threat by providing a customer-specific identifier in the trust policy.
Trap categories for this question
Similar concept trap
This would work technically, but it is far too broad and does not protect against confused deputy abuse. Any principal in Account B that can reach the role could potentially use it, which violates the customer’s security requirement.
Detailed technical explanation
How to think about this question
The `sts:ExternalId` condition key in the trust policy acts as a shared secret between the customer and the vendor, ensuring the role assumption is tied to a specific business relationship. Under the hood, AWS STS includes the external ID in the `AssumeRole` request, and the trust policy evaluates it against the condition; if the ID is missing or mismatched, the request is denied. This is critical in multi-tenant SaaS architectures where a single vendor account must securely access many customer resources without cross-tenant leakage.
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
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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: Allow only the vendor’s specific IAM principal to assume the role and require a unique sts:ExternalId condition. — Option B is correct because the `sts:ExternalId` condition is specifically designed to prevent the confused deputy problem in cross-account role assumptions. By requiring a unique external ID that only the customer knows, the customer ensures that the vendor's automation can only assume the role when acting on behalf of that specific customer, even if multiple customers use the same vendor software.
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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Same concept, more angles
3 more ways this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A software vendor in Account B must assume a role in Account A to process support tickets. Security wants to prevent confused deputy attacks. Which two configurations are required for this access pattern to work safely? Select two.
medium- ✓ A.Require a specific sts:ExternalId value in the role trust policy in Account A.
- ✓ B.Make sure the vendor includes that same ExternalId when calling sts:AssumeRole.
- C.Share long-term access keys from Account A with the vendor.
- D.Attach a permissions boundary to the role to satisfy the ExternalId requirement.
- E.Allow sts:GetSessionToken instead of sts:AssumeRole in the trust policy.
Why A: Option A is correct because requiring a specific sts:ExternalId value in the role trust policy in Account A is a standard AWS mechanism to prevent the confused deputy problem. The ExternalId acts as a unique secret that the vendor must provide when assuming the role, ensuring that the role is assumed only for the intended purpose and not by a malicious third party.
Variation 2. Based on the exhibit, a workload in Account B must assume a role in Account A. Security requires that only the specific role arn:aws:iam::444455556666:role/PipelineExecRole can assume it, and only when the caller supplies the external ID acct-b-prod-7788. Which change best satisfies the requirement with the least privilege?
hard- A.Keep the root principal and add an aws:PrincipalTag condition in the trust policy to require the tag acct-b-prod-7788.
- ✓ B.Replace the principal with arn:aws:iam::444455556666:role/PipelineExecRole and add a StringEquals condition on sts:ExternalId = acct-b-prod-7788.
- C.Attach a permission boundary to the role in Account A so that only PipelineExecRole can use it.
- D.Add an SCP in Account B that allows sts:AssumeRole only for PipelineExecRole.
Why B: Option B is correct because it explicitly restricts the trust policy principal to the specific IAM role ARN `arn:aws:iam::444455556666:role/PipelineExecRole` and adds a `StringEquals` condition on `sts:ExternalId` set to `acct-b-prod-7788`. This satisfies the security requirement by ensuring only that exact role can assume the role in Account A, and only when the correct external ID is provided, following the principle of least privilege.
Variation 3. Account A has an IAM role named FinanceDataRole that is assumed by a principal in Account B. The role’s trust policy includes a condition requiring sts:ExternalId to equal "Fin-2026-Q2". A developer in Account B calls AssumeRole but receives an error: AccessDenied: ExternalId mismatch. The security team requires that you do not remove the ExternalId condition. What is the correct remediation?
medium- A.Add kms:Decrypt to the developer’s IAM policy so KMS can validate the ExternalId during AssumeRole.
- ✓ B.Update the AssumeRole call in Account B to include sts:ExternalId="Fin-2026-Q2" exactly as required.
- C.Increase the role’s MaxSessionDuration to reduce authentication failures.
- D.Remove the ExternalId condition from the trust policy to allow all AssumeRole requests.
Why B: The error 'AccessDenied: ExternalId mismatch' occurs because the AssumeRole API call from Account B does not include the required sts:ExternalId parameter. The trust policy on the FinanceDataRole explicitly requires this parameter to match 'Fin-2026-Q2' as a security measure to prevent the confused deputy problem. Option B is correct because the developer must pass the exact ExternalId value in the AssumeRole request to satisfy the condition and successfully assume the role.
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Last reviewed: Jun 11, 2026
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