- A
Specify the exact vendor role ARN as the trusted principal in the role trust policy.
The trust policy should name only the specific vendor role that is allowed to assume the role in your account. Restricting the principal minimizes the trust boundary and prevents unrelated identities from attempting the assumption path.
- B
Require an external ID in the trust policy conditions.
An external ID helps prevent confused-deputy attacks because the vendor must present a value that your account and the vendor agreed on. That prevents another customer from tricking the vendor into using the same role on their behalf.
- C
Require sts:SourceIdentity when the vendor assumes the role.
SourceIdentity is recorded in CloudTrail and becomes part of the assumed-role session context. Requiring it gives investigators a way to trace the session back to the individual vendor user or workload that initiated the role assumption.
- D
Use a wildcard principal and rely on the S3 bucket policy to narrow access later.
Why wrong: A wildcard principal makes the trust policy too broad and expands who can attempt to assume the role. The trust policy should be the first and tightest control, while the bucket policy should only govern what an already-authorized session can do after assumption.
- E
Give the vendor long-term IAM user credentials in your account for easier auditing.
Why wrong: Long-term credentials increase operational and security risk and do not provide the same safety properties as temporary role sessions. Cross-account access should use STS and temporary credentials, not shared long-lived IAM users in the customer account.
Quick Answer
The answer is to require sts:SourceIdentity when the vendor assumes the role, specify the exact vendor role ARN as the trusted principal, and use sts:ExternalId in the trust policy. These three controls work together to prevent confused deputy attacks by ensuring that every assumed session is traceable back to an individual vendor user in CloudTrail. The sts:SourceIdentity condition forces the vendor to pass a unique identifier for each user, making every API call attributable, while the exact role ARN limits the trust boundary to only that specific role, and the sts:ExternalId acts as a shared secret that prevents a malicious third party from tricking your account into granting access. On the SAA-C03 exam, this scenario tests your understanding of the confused deputy problem in cross-account role assumption, where an attacker could exploit a role’s trust policy to impersonate a legitimate vendor. A common trap is thinking that simply trusting the vendor’s account is enough, but without these three conditions, any role in that account could assume your role. Memory tip: “Source, ARN, External” — like a three-legged stool for secure cross-account trust.
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 third-party payroll vendor in another AWS account must assume a role in your account to write a daily settlement file to Amazon S3. You want to prevent confused-deputy attacks and make every assumed session traceable in CloudTrail back to an individual vendor user. Which three trust-policy or session controls should be used? Select three.
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
Specify the exact vendor role ARN as the trusted principal in the role trust policy.
Option A is correct because specifying the exact vendor role ARN as the trusted principal in the trust policy ensures that only that specific role in the vendor's account can assume the role, preventing any other entity from impersonating the vendor. This is a key control to limit the trust boundary and avoid confused-deputy attacks.
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.
- ✓
Specify the exact vendor role ARN as the trusted principal in the role trust policy.
Why this is correct
The trust policy should name only the specific vendor role that is allowed to assume the role in your account. Restricting the principal minimizes the trust boundary and prevents unrelated identities from attempting the assumption path.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Require an external ID in the trust policy conditions.
Why this is correct
An external ID helps prevent confused-deputy attacks because the vendor must present a value that your account and the vendor agreed on. That prevents another customer from tricking the vendor into using the same role on their behalf.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Require sts:SourceIdentity when the vendor assumes the role.
Why this is correct
SourceIdentity is recorded in CloudTrail and becomes part of the assumed-role session context. Requiring it gives investigators a way to trace the session back to the individual vendor user or workload that initiated the role assumption.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a wildcard principal and rely on the S3 bucket policy to narrow access later.
Why it's wrong here
A wildcard principal makes the trust policy too broad and expands who can attempt to assume the role. The trust policy should be the first and tightest control, while the bucket policy should only govern what an already-authorized session can do after assumption.
- ✗
Give the vendor long-term IAM user credentials in your account for easier auditing.
Why it's wrong here
Long-term credentials increase operational and security risk and do not provide the same safety properties as temporary role sessions. Cross-account access should use STS and temporary credentials, not shared long-lived IAM users in the customer account.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think a bucket policy alone can control role assumption, but it cannot—the trust policy is the only mechanism to restrict which external principals can assume a role, and confused-deputy protections require explicit conditions like external ID and source identity.
Detailed technical explanation
How to think about this question
The confused-deputy problem occurs when a principal in one account tricks a role in another account into performing actions on its behalf. Using sts:SourceIdentity (Option C) forces the vendor to pass a unique identifier for each user when calling AssumeRole, which is then logged in CloudTrail, enabling traceability to individual vendor users. The external ID (Option B) is a shared secret that must match between the trust policy and the AssumeRole request, preventing the vendor from using the role from a different context or client.
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: Specify the exact vendor role ARN as the trusted principal in the role trust policy. — Option A is correct because specifying the exact vendor role ARN as the trusted principal in the trust policy ensures that only that specific role in the vendor's account can assume the role, preventing any other entity from impersonating the vendor. This is a key control to limit the trust boundary and avoid confused-deputy attacks.
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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