IAM policy evaluation logic in AWS follows a specific order to determine whether a request is allowed or denied. Understanding this logic is crucial for SysOps Administrators managing security and compliance. The evaluation process follows these key principles: First, by default, all requests are i…IAM policy evaluation logic in AWS follows a specific order to determine whether a request is allowed or denied. Understanding this logic is crucial for SysOps Administrators managing security and compliance. The evaluation process follows these key principles: First, by default, all requests are implicitly denied. AWS starts with the assumption that nothing is permitted unless explicitly allowed. This provides a secure foundation where access must be granted through policies. Second, an explicit allow in a policy overrides the implicit deny. When you attach a policy that grants permissions, those specific actions become permitted for the principal. Third, and most importantly, an explicit deny always takes precedence over any allow. If any policy contains a deny statement that matches the request, access is refused regardless of other policies that might allow it. The evaluation considers multiple policy types in this order: Service Control Policies (SCPs) at the organization level, resource-based policies attached to the target resource, identity-based policies attached to the IAM user or role, IAM permissions boundaries, and session policies for temporary credentials. For a request to succeed, all applicable policy types must permit the action. For cross-account access, both the source account identity policy and the destination account resource policy must allow the request. When evaluating policies, AWS examines the Principal, Action, Resource, and Conditions elements. Condition keys enable fine-grained control based on factors like IP address, time, MFA status, or tags. Best practices include using the principle of least privilege, regularly auditing permissions with IAM Access Analyzer, and testing policies using the IAM Policy Simulator before deployment. Understanding this evaluation logic helps administrators troubleshoot access issues and design secure, compliant permission structures that meet organizational requirements while maintaining operational efficiency.
IAM Policy Evaluation Logic - Complete Guide for AWS SysOps Administrator Associate
Why IAM Policy Evaluation Logic is Important
Understanding IAM policy evaluation logic is critical for AWS SysOps Administrators because it determines whether access requests are allowed or denied. Misunderstanding this logic can lead to security vulnerabilities or overly restrictive policies that block legitimate access. For the exam, this topic tests your ability to troubleshoot access issues and design secure permission structures.
What is IAM Policy Evaluation Logic?
IAM policy evaluation logic is the process AWS uses to determine whether to allow or deny a request when a principal (user, role, or service) attempts to perform an action on an AWS resource. AWS evaluates all applicable policies and makes an authorization decision based on a specific order of precedence.
How IAM Policy Evaluation Works
Step 1: Default Denial All requests are denied by default. This is the starting point for every evaluation.
Step 2: Evaluate All Applicable Policies AWS gathers and evaluates all policies that apply to the request, including: - Identity-based policies (attached to users, groups, or roles) - Resource-based policies (attached to resources like S3 buckets) - IAM permissions boundaries - Organizations Service Control Policies (SCPs) - Session policies (for assumed roles)
Step 3: Explicit Deny Check If any policy contains an explicit deny that matches the request, the request is denied. Explicit denies always take precedence over allows.
Step 4: Explicit Allow Check If there is an explicit allow and no explicit deny, the request is allowed.
Step 5: Implicit Deny If there is no explicit allow, the request is denied (implicit deny).
Service Control Policies (SCPs): Set maximum permissions for accounts in AWS Organizations. They do not grant permissions but restrict what identity-based policies can allow.
Permissions Boundaries: Set maximum permissions for an IAM entity. The effective permissions are the intersection of identity-based policies and permissions boundaries.
Resource-based Policies: Can grant cross-account access. If a resource-based policy grants access and there is no explicit deny, access may be allowed even when identity-based policies do not grant permission (within the same account).
Cross-Account Access Evaluation
For cross-account access, both the identity-based policy in the source account AND the resource-based policy in the destination account must allow the action. Both sides must grant permission for access to succeed.
Exam Tips: Answering Questions on IAM Policy Evaluation Logic
1. Remember: Explicit Deny Always Wins - If any policy has a deny statement that matches the request, access is blocked regardless of other allow statements.
2. Default is Deny - If a question describes a scenario where no policies grant access, the answer is always deny.
3. Look for Policy Conflicts - When multiple policies exist, check for conflicting statements. Denies override allows.
4. Permissions Boundaries Limit Permissions - Effective permissions are the intersection of identity policies and boundaries. If something is not allowed in the boundary, it cannot be done.
5. SCPs Apply to Member Accounts - SCPs do not affect the management account but restrict all other accounts in the organization.
6. Cross-Account Requires Both Sides - For cross-account scenarios, verify that both the source and destination policies allow the action.
7. Watch for NotAction and NotResource - These inverse statements can be tricky. Understand that NotAction means all actions except those listed.
8. Condition Keys Matter - Policies may have conditions that must be met. Check if the request context satisfies all conditions.
9. Session Policies Further Restrict - When assuming a role with session policies, the effective permissions are limited to the intersection of the role policy and session policy.
10. Practice Elimination - In complex scenarios, eliminate answers that suggest access is granted when an explicit deny exists, or when no allow statement covers the requested action.