Understanding Cedar policies - Amazon Bedrock AgentCore
Example policyPolicy structurePolicy effectsDefault denyAuthorization evaluationPolicy independencePolicy evaluation algorithm

Understanding Cedar policies

Policy in AgentCore uses Cedar policies to control access to AgentCore Gateway tools. This section explains Cedar policy structure, evaluation semantics, and key concepts.

Example policy

Consider a refund processing tool with these requirements:

  • Only the user "John" can process refunds

  • Refunds are limited to $500 or less

The Cedar policy that enforces these requirements:


permit(
  principal is AgentCore::OAuthUser,
  action == AgentCore::Action::"RefundTool__process_refund",
  resource == AgentCore::Gateway::"arn:aws:bedrock-agentcore:your-region:your-account-id:gateway/refund-gateway"
)
when {
  principal.hasTag("username") &&
  principal.getTag("username") == "John" &&
  context.input.amount < 500
};

This policy allows refund processing only when the user is "John" and the refund amount is less than $500.

Policy structure

Cedar policies consist of three main components:

  1. Effect - Determines whether to allow or deny access (permit or forbid)

  2. Scope - Specifies the principal, action, and resource the policy applies to

  3. Condition - Defines additional logic that must be satisfied (when or unless) and can refer to the tool parameters (through the context) and the OAuth token (through the tags)

Policy effects

Cedar policies use two effects to control access:

  • permit - Allows the action to proceed

  • forbid - Denies the action

Default deny

All actions are denied by default. If no policies match a request, Cedar returns DENY. You must explicitly write permit policies to allow actions.

Authorization evaluation

Cedar uses a forbid-overrides-permit evaluation model:

  1. Cedar evaluates all policies that apply to the request

  2. If any forbid policy matches, the result is DENY

  3. If at least one permit policy matches and no forbid policies does, the result is ALLOW

  4. If no policies match, the result is DENY (default deny)

Policy independence

Each Cedar policy evaluates independently. A policy's evaluation depends only on:

  • The scope (principal, action, resource)

  • The context and tags

Policies do not reference or depend on other policies.

Policy evaluation algorithm

When a request is evaluated, the policy engine determines the authorization decision using the following algorithm:

  1. If any forbid policy matches the request, the decision is DENY.

  2. If no forbid policy matches the request and at least one permit policy matches, the decision is ALLOW.

  3. If neither forbid nor permit policies match the request, the decision is DENY.

This evaluation model enforces a default deny posture.

A forbid policy can never result in an ALLOW decision. The unless clause on a forbid policy specifies conditions under which that forbid policy does not apply; it does not grant permission and does not override a matching permit policy.