Policy evaluation logic in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the first step in AWS policy evaluation logic?

AWS starts by checking for any explicit Deny statements in the policies attached to the user or resource.

beginner

What happens if there is no explicit Deny and no Allow in AWS policy evaluation?

The default is to deny access if no explicit Allow is found.

intermediate

How does AWS handle multiple policies attached to a user or resource during evaluation?

AWS evaluates all policies together. If any policy explicitly denies, the request is denied. If none deny and at least one allows, the request is allowed.

beginner

What is the effect of an explicit Deny in AWS policy evaluation?

An explicit Deny always overrides any Allow, blocking the requested action regardless of other policies.

intermediate

Why is it important to understand AWS policy evaluation logic?

Understanding the logic helps you design policies that correctly allow or block actions, avoiding unintended access or denial.

In AWS policy evaluation, what happens if a policy explicitly denies an action?

A The action is allowed if another policy allows it.

B The action is allowed by default.

C The evaluation stops and asks for user confirmation.

D The action is denied regardless of other policies.

An explicit Deny always overrides any Allow, so the action is denied.

If no policy explicitly allows or denies an action in AWS, what is the result?

A The action is allowed by default.

B The action is denied by default.

C The action is allowed only if the user is an admin.

D The action is logged but not executed.

AWS denies actions by default if there is no explicit Allow.

When multiple policies apply, how does AWS decide to allow an action?

A If any policy denies, the action is allowed.

B Only the first policy is evaluated.

C If any policy allows and none deny, the action is allowed.

D Policies are ignored if the user is root.

AWS allows the action if at least one policy allows it and none explicitly deny it.

What is the role of resource-based policies in AWS policy evaluation?

A They are evaluated along with user policies.

B They are ignored if user policies exist.

C They override all user policies.

D They only apply to AWS root users.

Resource-based policies are evaluated together with user policies during access decisions.

Which of the following best describes AWS policy evaluation logic?

A Deny overrides Allow.

B Only one policy is evaluated per request.

C Policies are evaluated randomly.

D Allow overrides Deny.

Deny always overrides Allow in AWS policy evaluation.

Explain the order of evaluation in AWS policy evaluation logic.

Think about how AWS decides to allow or deny an action step by step.

Describe why an explicit Deny is important in AWS policies.

Consider the strongest rule in policy evaluation.

Practice

1. What happens if an AWS IAM policy has both an explicit Allow and an explicit Deny for the same action?

easy

A. The explicit Deny always overrides the Allow.

B. The Allow always overrides the Deny.

C. The action is allowed only if the user is an administrator.

D. The action is denied only if there is a condition attached.

Solution

1. Step 1: Understand explicit Deny effect

   In AWS IAM, an explicit Deny always takes priority over any Allow for the same action.

2. Step 2: Apply policy evaluation logic

   Even if a policy allows an action, if another policy explicitly denies it, the Deny wins and the action is blocked.

3. Final Answer:

   The explicit Deny always overrides the Allow. -> Option A

4. Quick Check:

   Explicit Deny > Allow [OK]

Hint: Remember: Deny always beats Allow in AWS policies [OK]

Common Mistakes:

• Thinking Allow can override Deny
• Ignoring explicit Deny effect
• Assuming conditions affect Deny priority

2. Which of the following is the correct JSON syntax to allow the s3:ListBucket action on a bucket named my-bucket?

easy

A. {\"Effect\": \"Deny\", \"Action\": \"s3:ListBucket\", \"Resource\": \"arn:aws:s3:::my-bucket\"}

B. {\"Effect\": \"Allow\", \"Action\": [\"s3:ListBucket\"], \"Resource\": \"arn:aws:s3:::my-bucket\"}

C. {\"Effect\": \"Allow\", \"Action\": \"s3:ListBucket\", \"Resource\": \"my-bucket\"}

D. {\"Effect\": \"Allow\", \"Action\": \"ListBucket\", \"Resource\": \"arn:aws:s3:::my-bucket\"}

Solution

1. Step 1: Check Action format

   The Action field must be a string or an array of strings. Using an array is valid and recommended for multiple actions.

2. Step 2: Verify Resource ARN format

   The Resource must be the full ARN: arn:aws:s3:::my-bucket for the bucket itself.

3. Final Answer:

   Action as array and correct ARN Resource -> Option B

4. Quick Check:

   Action array + correct ARN = D [OK]

Hint: Use full ARN and array for actions to avoid syntax errors [OK]

Common Mistakes:

• Using bucket name instead of ARN in Resource
• Omitting array brackets for multiple actions
• Using action name without service prefix

3. Given this policy snippet:

{
  "Effect": "Allow",
  "Action": "ec2:StartInstances",
  "Resource": "*",
  "Condition": {
    "IpAddress": {"aws:SourceIp": "203.0.113.0/24"}
  }
}

What happens if a user tries to start an EC2 instance from IP 198.51.100.10?

medium

A. The action is denied because the IP does not match the condition.

B. The action is allowed because the Effect is Allow.

C. The action is allowed only if the user has another policy allowing it.

D. The action is denied only if there is an explicit Deny policy.

Solution

1. Step 1: Understand Condition effect

   The policy allows the action only if the request comes from IPs in 203.0.113.0/24 range.

2. Step 2: Check IP address

   The user's IP 198.51.100.10 is outside the allowed range, so the condition fails.

3. Final Answer:

   The action is denied because the IP does not match the condition. -> Option A

4. Quick Check:

   Condition IP mismatch = Deny [OK]

Hint: Conditions restrict Allow; mismatch means Deny [OK]

Common Mistakes:

• Ignoring condition and assuming Allow always works
• Confusing explicit Deny with condition-based Deny
• Assuming multiple policies needed to allow

4. You have two policies attached to a user:
Policy 1: Allows s3:GetObject on bucket my-bucket.
Policy 2: Denies s3:GetObject on bucket my-bucket if the request is from outside office IP range.

The user tries to get an object from home IP. What is the result?

medium

A. The request is allowed because Policy 1 allows it.

B. The request is allowed only if the user is in the admin group.

C. The request is denied only if there is a service outage.

D. The request is denied because Policy 2 explicitly denies it from outside IPs.

Solution

1. Step 1: Identify explicit Deny with condition

   Policy 2 denies the action if the IP is outside the office range, which applies here.

2. Step 2: Apply evaluation logic

   Explicit Deny overrides any Allow, so the request is denied.

3. Final Answer:

   The request is denied because Policy 2 explicitly denies it from outside IPs. -> Option D

4. Quick Check:

   Explicit Deny with condition blocks request [OK]

Hint: Explicit Deny with condition beats Allow always [OK]

Common Mistakes:

• Ignoring condition in Deny policy
• Assuming Allow always wins
• Thinking user group affects Deny priority

5. You want to create a policy that allows ec2:StopInstances only during business hours (9 AM to 5 PM UTC) and denies it otherwise. Which policy logic correctly enforces this?

hard

A. Only use Deny with condition outside 9-17 UTC, no Allow needed.

B. Allow ec2:StopInstances with condition "DateGreaterThan": {"aws:CurrentTime": "09:00:00Z"}, no Deny needed.

C. Allow ec2:StopInstances unconditionally, and add a Deny with condition outside 9-17 UTC.

D. Allow ec2:StopInstances with condition for 9-17 UTC, and Deny unconditionally.

Solution

1. Step 1: Understand Deny override with time condition

   Unconditional Allow permits ec2:StopInstances, but explicit Deny applies outside 9-17 UTC overriding the Allow.

2. Step 2: Verify business hours enforcement

   During 9 AM-5 PM UTC: Deny condition false -> action allowed. Outside: Deny true -> denied.

3. Final Answer:

   Allow unconditionally, and add a Deny with condition outside 9-17 UTC. -> Option C

4. Quick Check:

   Allow + Deny conditions enforce time limits [OK]

Hint: Unconditional Allow + conditional Deny outside business hours [OK]

Common Mistakes:

• Relying only on Allow conditions without Deny
• Using unconditional Deny that blocks all
• Missing time range in conditions