- A
Add an sts:ExternalId condition to the role trust policy that must match the unique external ID you provide to the vendor.
The sts:ExternalId condition is a common protection against confused-deputy scenarios in cross-account role assumption. It ensures that only principals who know the unique external ID can successfully assume the role. This mitigates a third party tricking the vendor’s identity into assuming your role, even if they can call AssumeRole.
- B
Require the vendor to use the same MFA device serial number as your internal administrators in the trust policy.
Why wrong: Trust policies can check conditions related to MFA in some contexts, but matching your internal MFA device serial to an external vendor is impractical and not the primary confused-deputy mitigation. External ID was designed specifically for this cross-account vendor role assumption pattern.
- C
Remove the role’s permissions policy and rely only on the S3 bucket policy to validate the caller.
Why wrong: Relying solely on the bucket policy does not address the confused-deputy risk in role assumption. Also, a missing permissions policy may break legitimate access. External ID is the relevant trust policy mitigation for limiting which assumers can obtain credentials.
- D
Allow sts:AssumeRole from the vendor account root principal without restricting to the vendor’s specific IAM role.
Why wrong: Allowing root principal broadly increases risk and does not mitigate confused-deputy attacks. The trust policy should be as specific as possible, and External ID provides a targeted protection against attackers misusing the vendor’s access.
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 needs temporary access to an S3 bucket in your AWS account to read customer exports. The vendor will assume an IAM role you created. During integration testing, the vendor reports that their AssumeRole requests succeed, but your security team is concerned about the possibility of confused-deputy attacks. Which trust policy approach most directly mitigates this risk?
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
Add an sts:ExternalId condition to the role trust policy that must match the unique external ID you provide to the vendor.
Option A is correct because the `sts:ExternalId` condition in the trust policy forces the vendor to include a unique external ID in their `AssumeRole` API call. This prevents a confused-deputy attack by ensuring that the role can only be assumed when the caller provides the exact external ID you have pre-shared, thereby verifying the intended purpose of the cross-account access.
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.
- ✓
Add an sts:ExternalId condition to the role trust policy that must match the unique external ID you provide to the vendor.
Why this is correct
The sts:ExternalId condition is a common protection against confused-deputy scenarios in cross-account role assumption. It ensures that only principals who know the unique external ID can successfully assume the role. This mitigates a third party tricking the vendor’s identity into assuming your role, even if they can call AssumeRole.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Require the vendor to use the same MFA device serial number as your internal administrators in the trust policy.
Why it's wrong here
Trust policies can check conditions related to MFA in some contexts, but matching your internal MFA device serial to an external vendor is impractical and not the primary confused-deputy mitigation. External ID was designed specifically for this cross-account vendor role assumption pattern.
- ✗
Remove the role’s permissions policy and rely only on the S3 bucket policy to validate the caller.
Why it's wrong here
Relying solely on the bucket policy does not address the confused-deputy risk in role assumption. Also, a missing permissions policy may break legitimate access. External ID is the relevant trust policy mitigation for limiting which assumers can obtain credentials.
- ✗
Allow sts:AssumeRole from the vendor account root principal without restricting to the vendor’s specific IAM role.
Why it's wrong here
Allowing root principal broadly increases risk and does not mitigate confused-deputy attacks. The trust policy should be as specific as possible, and External ID provides a targeted protection against attackers misusing the vendor’s access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think MFA or bucket policies are sufficient for cross-account security, but the confused-deputy attack is specifically mitigated by the `sts:ExternalId` condition, not by authentication factors or resource-based policies alone.
Trap categories for this question
Similar concept trap
Trust policies can check conditions related to MFA in some contexts, but matching your internal MFA device serial to an external vendor is impractical and not the primary confused-deputy mitigation. External ID was designed specifically for this cross-account vendor role assumption pattern.
Detailed technical explanation
How to think about this question
The `sts:ExternalId` condition key is evaluated during the `AssumeRole` API call and must match the value in the request context. Under the hood, AWS STS generates temporary credentials only if the external ID in the request matches the one in the trust policy, ensuring that the role is assumed only for the intended third-party use case. In a real-world scenario, if the vendor’s account is compromised, an attacker could attempt to assume your role, but without the correct external ID, the request is denied—even if the attacker has valid IAM credentials in the vendor account.
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: Add an sts:ExternalId condition to the role trust policy that must match the unique external ID you provide to the vendor. — Option A is correct because the `sts:ExternalId` condition in the trust policy forces the vendor to include a unique external ID in their `AssumeRole` API call. This prevents a confused-deputy attack by ensuring that the role can only be assumed when the caller provides the exact external ID you have pre-shared, thereby verifying the intended purpose of the cross-account access.
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.
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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