SAA-C03 Design Secure Architectures Practice Question
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?
⚠ Common exam trap
It's easy for candidates to think MFA (Option B) or bucket policies (Option C) are sufficient for cross-account access security, but they fail to address the specific confused-deputy vector that `sts:ExternalId` is designed to block.
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.
Adding an `sts:ExternalId` condition to the role 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 presents the specific external ID you control, even if the vendor's account is compromised or used by a different AWS service.
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.
- ✗
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.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAA-C03 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAA-C03 exam.