- A
Provide the correct --external-id value (b-7f9a) in the AssumeRole call.
The trust policy requires sts:ExternalId to equal b-7f9a. If the caller does not supply the matching external ID, STS fails the trust-policy condition and denies AssumeRole. Supplying --external-id b-7f9a satisfies the condition.
- B
Add kms:Decrypt permissions to Account B’s IAM user because trust policy failures are KMS related.
Why wrong: The error is about trust policy evaluation for AssumeRole (sts:ExternalId mismatch). KMS permissions are not involved in evaluating that trust condition.
- C
Remove the ExternalId condition from the trust policy so any caller can assume the role.
Why wrong: This would weaken security by removing a deliberate external-party verification control. The scenario’s stated fix is to satisfy the existing trust condition rather than eliminate it.
- D
Use AssumeRoleWithSAML instead of AssumeRole so ExternalId is not required.
Why wrong: AssumeRoleWithSAML still results in STS evaluating the same role trust policy conditions. If the trust policy requires sts:ExternalId, it must be satisfied regardless of the AssumeRole API used.
SAA-C03 Design Secure Architectures Practice Question
This SAA-C03 practice question tests your understanding of design secure architectures. 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.
Account A hosts a role named AppReadRole. Account B needs to access it using STS AssumeRole. Account A’s role trust policy includes this condition: - StringEquals: { "sts:ExternalId": "b-7f9a" } When Account B runs: aws sts assume-role --role-arn arn:aws:iam::111111111111:role/AppReadRole --role-session-name test the call fails with: "AccessDenied: ExternalId mismatch". What should Account B change?
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
Provide the correct --external-id value (b-7f9a) in the AssumeRole call.
The error 'AccessDenied: ExternalId mismatch' occurs because the trust policy on Account A's role requires an `sts:ExternalId` condition with the value `b-7f9a`, but Account B's `aws sts assume-role` command did not include the `--external-id` parameter. By providing the correct `--external-id b-7f9a` in the call, Account B satisfies the condition, allowing the role assumption to succeed. This is a standard security mechanism to prevent the confused deputy problem.
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.
- ✓
Provide the correct --external-id value (b-7f9a) in the AssumeRole call.
Why this is correct
The trust policy requires sts:ExternalId to equal b-7f9a. If the caller does not supply the matching external ID, STS fails the trust-policy condition and denies AssumeRole. Supplying --external-id b-7f9a satisfies the condition.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Add kms:Decrypt permissions to Account B’s IAM user because trust policy failures are KMS related.
Why it's wrong here
The error is about trust policy evaluation for AssumeRole (sts:ExternalId mismatch). KMS permissions are not involved in evaluating that trust condition.
- ✗
Remove the ExternalId condition from the trust policy so any caller can assume the role.
Why it's wrong here
This would weaken security by removing a deliberate external-party verification control. The scenario’s stated fix is to satisfy the existing trust condition rather than eliminate it.
- ✗
Use AssumeRoleWithSAML instead of AssumeRole so ExternalId is not required.
Why it's wrong here
AssumeRoleWithSAML still results in STS evaluating the same role trust policy conditions. If the trust policy requires sts:ExternalId, it must be satisfied regardless of the AssumeRole API used.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may think the error is due to missing permissions (like KMS) or that changing the API method (SAML) avoids the condition, when in fact the fix is simply to include the required `--external-id` parameter in the AssumeRole call.
Trap categories for this question
Scenario analysis trap
This would weaken security by removing a deliberate external-party verification control. The scenario’s stated fix is to satisfy the existing trust condition rather than eliminate it.
Detailed technical explanation
How to think about this question
The `sts:ExternalId` condition is used to prevent the confused deputy problem, where a malicious service tricks a role into accessing resources it shouldn't. Under the hood, AWS evaluates the condition against the `externalId` parameter in the `AssumeRole` API request; if omitted or mismatched, the call fails with AccessDenied. In real-world scenarios, this is common when a third-party service (Account B) needs to access resources in your account (Account A) — the ExternalId acts as a shared secret to ensure only authorized clients can assume the role.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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: Provide the correct --external-id value (b-7f9a) in the AssumeRole call. — The error 'AccessDenied: ExternalId mismatch' occurs because the trust policy on Account A's role requires an `sts:ExternalId` condition with the value `b-7f9a`, but Account B's `aws sts assume-role` command did not include the `--external-id` parameter. By providing the correct `--external-id b-7f9a` in the call, Account B satisfies the condition, allowing the role assumption to succeed. This is a standard security mechanism to prevent the confused deputy problem.
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
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.
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