Policy evaluation logic in AWS - Step-by-Step Execution

Process Flow - Policy evaluation logic

Start: Request Received

↓

Check Explicit Deny?

Yes→Deny Access

No↓

Check Explicit Allow?

Yes→Allow Access

No↓

Default Deny

↓

End: Access Decision

The policy evaluation checks for explicit deny first, then explicit allow, and defaults to deny if no rules match.

Execution Sample

AWS

Request: User wants to S3:GetObject
Policies:
- Deny S3:GetObject on bucket 'secret'
- Allow S3:GetObject on bucket 'public'

Evaluates if the user can get an object from S3 based on deny and allow policies.

Process Table

Step	Check	Condition	Result	Decision
1	Explicit Deny	Is action S3:GetObject on bucket 'secret'?	Yes	Deny Access
2	Explicit Allow	Skipped because deny found	N/A	Deny Access
3	Default Deny	Not reached	N/A	Deny Access

💡 Explicit deny found at step 1, so access is denied immediately.

Status Tracker

Variable	Start	After Step 1	After Step 2	Final
Access Decision	None	Deny	Deny	Deny

Key Moments - 2 Insights

Why does the evaluation stop after finding an explicit deny?

Because explicit deny always overrides any allow, the evaluation stops immediately to deny access as shown in step 1 of the execution_table.

What happens if no explicit deny or allow matches?

The default deny rule applies, so access is denied by default, as indicated in step 3 which is reached only if no explicit rules match.

Visual Quiz - 3 Questions

Look at the execution_table, what is the access decision after step 1?

A Allow

B Deny

C Undecided

D Error

💡 Hint

Check the 'Decision' column in row for step 1 in execution_table.

At which step does the evaluation skip checking explicit allow?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Result' column for step 2 in execution_table.

If the explicit deny policy was removed, what would be the final decision for the same request?

A Allow

B Deny

C Undecided

D Error

💡 Hint

Refer to variable_tracker and consider what happens if step 1 deny is not found.

Concept Snapshot

Policy evaluation logic:
1. Check explicit deny first - if yes, deny immediately.
2. If no deny, check explicit allow - if yes, allow.
3. If neither, default deny.
This ensures security by prioritizing deny rules.

Full Transcript

When a request is made, AWS policy evaluation starts by checking if any explicit deny matches the request. If yes, access is denied immediately. If no explicit deny is found, it checks for explicit allow. If an allow matches, access is granted. If neither deny nor allow matches, access is denied by default. This logic ensures that deny rules always override allow rules, providing a secure default. The execution table shows this step-by-step with a request to get an object from an S3 bucket. The variable tracker shows the access decision changing from none to deny after the explicit deny is found. Key moments clarify why evaluation stops on deny and what happens if no rules match. The visual quiz tests understanding of these steps.

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