Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.
For DVA-C02 candidates, the confusion often lies in how IAM policy evaluation precedence works when multiple credentials are available on an EC2 instance. In production, this is about understanding exactly which credentials are honored by the SDK when both an instance profile role and static credentials exist, and how explicit denies impact effective permissions. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
TechPulse Inc. runs an internal analytics platform on Amazon EC2 instances. Each instance is associated with an IAM role that explicitly denies all Amazon S3 API actions for security reasons. However, the EC2 instances also have a local credentials file containing static IAM access key and secret key with full administrative access to S3.
The Requirement: #
Given that both an IAM role (with an explicit deny) and static credentials (with full admin permissions) are available to the application running on the EC2 instance, which statement accurately describes the instance’s ability to perform Amazon S3 actions?
The Options #
- A) The instance will only be able to list all S3 buckets.
- B) The instance will only be able to list the contents of one S3 bucket at a time.
- C) The instance will be able to perform all actions on any S3 bucket.
- D) The instance will not be able to perform any S3 action on any S3 bucket.
Google adsense #
leave a comment:
Correct Answer #
D) The instance will not be able to perform any S3 action on any S3 bucket.
Quick Insight: The Developer Imperative #
When multiple authentication methods are present, the AWS SDK and CLI give precedence to the first set of credentials in the search order—starting with the Instance Profile role over static credentials. Also, IAM policies with explicit deny statements override any allow permissions, regardless of the credential source.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option D
The Winning Logic #
AWS SDKs use a well-defined credential provider chain on EC2 instances, which includes (in order): environment variables, credentials files, instance profile credentials. When multiple credentials coexist, the SDK attempts them in order until valid credentials are found. However, EC2 Instance Profile credentials are evaluated first by the SDK if local environment variables or profile overrides don’t exist.
In this scenario, the instance profile role explicitly denies all S3 API actions. IAM evaluation rules state that any explicit deny takes precedence, regardless of other policies or credentials attached elsewhere (including static credentials in the credentials file). Therefore, the explicit deny on the instance role prevents any S3 actions, even though static credentials allow full access. The SDK’s default behavior means the deny in the role wins, denying access entirely.
The Trap (Distractor Analysis) #
-
Why not A?
Listing buckets is an S3 API action—explicit deny blocks this access entirely; no partial privilege applies. -
Why not B?
This option misunderstands IAM evaluation and S3 API actions; IAM doesn’t grant partial bucket listing based on credentials used. -
Why not C?
This would be true if the static credentials were the only credentials in use and SDK honored them above the role, but explicit deny in the role cannot be overridden.
The Technical Blueprint #
# Credential provider chain precedence on EC2 (simplified)
AWS SDK Credential Lookup Order:
1) Environment Variables AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY
2) Shared Credentials File (~/.aws/credentials)
3) EC2 Instance Metadata Service (Instance Profile Role)
# IAM policy evaluation rule example:
# An Explicit Deny in any attached policy overrides Allow
The Comparative Analysis #
| Option | API Complexity | Performance Impact | Use Case / Explanation |
|---|---|---|---|
| A | Partial listing | Invalid scenario | Listing buckets is blocked by explicit deny |
| B | Partial listing | Invalid scenario | No IAM mechanism supports partial bucket view |
| C | Full access | Incorrect | Would contradict explicit deny override rules |
| D | Denied completely | Correct | Matches IAM explicit deny precedence rules |
Real-World Application (Practitioner Insight) #
Exam Rule #
“For the exam, always remember that explicit deny policies trump any other allows, regardless of credential source or order.”
Real World #
“In production, never mix static credentials with instance profiles on EC2 instances; it leads to unpredictable SDK credential resolution and complicates troubleshooting.”
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS DVA-C02 exam.