The answer is to modify the trust policy to specify the exact user ARN instead of the root ARN. This is the most secure way to restrict cross-account role assumption to a specific user because the trust policy in the target account explicitly defines which principals from the external account are allowed to assume the role. By replacing the root ARN with the specific IAM user ARN, you enforce a direct, granular trust boundary that cannot be bypassed by other users or roles in the external account. On the AWS Certified Security Specialty SCS-C02 exam, this concept tests your understanding of the difference between identity-based policies and resource-based trust policies—a common trap is confusing SCPs or the external account’s IAM policies with the trust policy’s control. Remember the memory tip: “Trust is local, permissions are global”—the trust policy in the production account is the only place that dictates who from outside can assume the role.
SCS-C02 Management and Security Governance Practice Question
This SCS-C02 practice question tests your understanding of management and security governance. 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 security engineer notices that an IAM role in the production account is being assumed by a user from another AWS account, which violates the principle of least privilege. The role's trust policy allows the root user of the external account. What is the MOST secure way to restrict access to only a specific user in the external account?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "least"
Why it matters: You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the trust policy to specify the exact user ARN instead of the root ARN.
Option C is correct because modifying the trust policy to include the specific user ARN is the most direct and secure approach. Option A is incorrect because SCPs do not affect cross-account access. Option B is incorrect because IAM policies on the role do not restrict who can assume it. Option D is incorrect because the external account's IAM policies do not control trust.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Apply an SCP to the external account to deny the sts:AssumeRole action.
Why it's wrong here
SCPs apply within an organization, not across different AWS organizations.
✗
Create an IAM policy in the external account that denies sts:AssumeRole for the role.
Why it's wrong here
External account policies do not affect the trust relationship.
✓
Modify the trust policy to specify the exact user ARN instead of the root ARN.
Why this is correct
Limits assumption to the specified user.
Clue confirmation
The clue word "least" in the question point toward this answer.
Related concept
Authentication checks who the user is.
✗
Add a condition to the role's permissions policy requiring a specific source IP.
Why it's wrong here
Does not restrict the assuming user identity.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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.
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related SCS-C02 questions on access control and AAA configuration.
Management and Security Governance — This question tests Management and Security Governance — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: Modify the trust policy to specify the exact user ARN instead of the root ARN. — Option C is correct because modifying the trust policy to include the specific user ARN is the most direct and secure approach. Option A is incorrect because SCPs do not affect cross-account access. Option B is incorrect because IAM policies on the role do not restrict who can assume it. Option D is incorrect because the external account's IAM policies do not control trust.
What should I do if I get this SCS-C02 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related SCS-C02 questions on access control and AAA configuration.
Are there clue words in this question I should notice?
Yes — watch for: "least". You want the option with minimum overhead, fewest steps, or lowest impact — not the most feature-rich or comprehensive answer.
What is the key concept behind this question?
Authentication checks who the user is.
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