AWS Certified Security – Specialty — Question 108

An Application team has requested a new AWS KMS master key for use with Amazon S3, but the organizational security policy requires separate master keys for different AWS services to limit blast radius.
How can an AWS KMS customer master key (CMK) be constrained to work with only Amazon S3?

Answer options

• A. Configure the CMK key policy to allow only the Amazon S3 service to use the kms:Encrypt action.
• B. Configure the CMK key policy to allow AWS KMS actions only when the kms:ViaService condition matches the Amazon S3 service name.
• C. Configure the IAM user's policy to allow KMS to pass a role to Amazon S3.
• D. Configure the IAM user's policy to allow only Amazon S3 operations when they are combined with the CMK.

Correct answer: B

Explanation

The correct answer is B because it specifically allows AWS KMS actions when the condition matches the Amazon S3 service, ensuring the key is limited to that service. Option A is incorrect because it does not restrict the key to only Amazon S3; it just limits the encrypt action. Option C is not relevant to constraining the CMK itself, and Option D does not limit the key usage to Amazon S3 but rather relates to IAM policies.