Using Identity-Based Policies With Amazon DynamoDB

In the realm of cloud services, the introduction of resource-based policies by Amazon DynamoDB signifies a notable advancement in access control and security management for its resources. These policies usher in a new era of granular and flexible permission settings, enabling a more nuanced approach to access management for DynamoDB resources, including tables and streams. This article is designed to guide you through the benefits and implementation of these policies, supplemented with practical examples.

Understanding Resource-Based Policies for DynamoDB

Before delving into implementation details, it's crucial to grasp the core concepts underpinning resource-based policies in Amazon DynamoDB. Unlike traditional IAM (Identity and Access Management) policies that are attached to users or roles, resource-based policies are directly attached to the DynamoDB resources themselves—be it tables or streams. This allows for specifying which IAM principals (users, roles, AWS accounts) can access a resource and the exact actions they are permitted to perform.

Resource-based policies streamline the management of permissions more directly and intuitively. They are particularly advantageous for specifying granular access controls and simplifying cross-account access, making them an essential tool for modern cloud architectures.

Benefits of Resource-Based Policies

Resource-based policies bring a host of advantages to the table, including but not limited to:

Granular Control

They enable precise specification of who can do what with a DynamoDB resource, allowing for detailed access control at the table or stream level. To illustrate the concept of granular control in resource-based policies for Amazon DynamoDB, let's consider a detailed policy example.

This example showcases how you can specify very precise access permissions for different IAM principals, focusing on a scenario where specific roles within an organization need different levels of access to a DynamoDB table.

Scenario Overview

In this scenario, our organization has a DynamoDB table named CustomerData. We want to grant different levels of access as follows: 

• Data Analysts should only be able to read data from the table. 
• Application Developers need to read and write data. 
• Auditors require the ability to scan the table to perform compliance checks.

Below is an example of a resource-based policy that implements these requirements by defining granular access controls for each role.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456789012:role/DataAnalysts"
      },
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:BatchGetItem",
        "dynamodb:Query",
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/CustomerData"
    },
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456789012:role/ApplicationDevelopers"
      },
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:BatchGetItem",
        "dynamodb:PutItem",
        "dynamodb:UpdateItem",
        "dynamodb:DeleteItem",
        "dynamodb:Query",
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/CustomerData"
    },
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456789012:role/Auditors"
      },
      "Action": [
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/CustomerData"
    }
  ]
}

The above policy is composed of three statements, each tailored to a specific role within the organization, granting varying levels of access to the CustomerData table.

The Data Analyst's role is to grant permission to perform read operations (GetItem, BatchGetItem, Query, and Scan). The Application Developer role allows both read and write access (GetItem, BatchGetItem, PutItem, UpdateItem, DeleteItem, Query, and Scan), enabling them to fully manage the data within the table.

The Auditor's role is limited to the Scan action, which is sufficient for compliance checks without allowing direct data manipulation.

Enhanced Security

By defining strict access parameters, resource-based policies help in adhering to the principle of least privilege, thus bolstering the security posture of your DynamoDB resources.

To illustrate an enhanced security control policy example for DynamoDB, let's consider a scenario where an organization wants to enforce strict access control to sensitive data within a DynamoDB table. The goal is to ensure that only certain roles within the organization can perform write operations, such as PutItem, UpdateItem, and DeleteItem, on the table, while a broader group of roles can perform read operations like GetItem and Scan. Additionally, the policy will restrict access based on specific conditions, such as requiring MFA (Multi-Factor Authentication) for write operations, to further enhance security.

Below is an example of the Enhanced Security Control Policy for DynamoDB.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "ReadAccessForAllAuthorizedRoles",
      "Effect": "Allow",
      "Principal": {
        "AWS": [
          "arn:aws:iam::123456789012:role/ReadOnlyRole",
          "arn:aws:iam::123456789012:role/DataAnalyst"
        ]
      },
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:Scan",
        "dynamodb:Query",
        "dynamodb:BatchGetItem"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/SensitiveDataTab"
    },
    {
      "Sid": "WriteAccessForSpecificRolesWithMFA",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456789012:role/DataCustodian"
      },
      "Action": [
        "dynamodb:PutItem",
        "dynamodb:UpdateItem",
        "dynamodb:DeleteItem"
      ],
      "Resource": "arn:aws:dynamodb:us-east-1:123456789012:table/SensitiveDataTab",
      "Condition": {
        "Bool": {
          "aws:MultiFactorAuthPresent": "true"
        }
      }
    }
  ]
}

In the above policy:

The first statement (Sid: ReadAccessForAllAuthorizedRoles) specifies that roles ReadOnlyRole and DataAnalyst is granted permission to perform read operations (GetItem, Scan, Query, BatchGetItem) on the Sensitive DataTable.

The second statement (Sid: WriteAccessForSpecificRolesWithMFA) grants the DataCustodian role permissions to perform write operations (PutItem, UpdateItem, DeleteItem) on the Sensitive DataTable, with the condition that MFA is enabled (aws:MultiFactorAuthPresent equals true).

This policy example showcases how DynamoDB's resource-based policies can be used to implement enhanced security controls by restricting access to sensitive data based on role and security conditions, such as requiring MFA for more sensitive write operations.

Cross-Account Access

These policies facilitate the sharing of DynamoDB resources across different AWS accounts in a more streamlined and secure manner, eliminating the need for complex setups.

In a scenario where an organization needs to allow a partner company to access a DynamoDB table for read-only operations, cross-account access control is crucial.

This example illustrates how to create a policy that permits an IAM role from a partner's AWS account to perform GetItem and Scan operations on a specific table in your account. The policy ensures that access is securely controlled, allowing only the specified actions from the trusted external account.

Below is an example of a Cross-Account Access Control Policy for DynamoDB:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "ReadOnlyAccessForPartnerAccount",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::PARTNER_ACCOUNT_ID:role/PartnerReadOnlyRole"
      },
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:YOUR_AWS_REGION:YOUR_ACCOUNT_ID:table/YourSha"
    }
  ]
}

In the above policy:

• "Sid": "ReadOnlyAccessForPartnerAccount" identifies this statement within the policy for easier management and review.
• "Effect": "Allow" grants the permissions specified in the statement.
• "Principal": { "AWS": "arn:aws:iam::PARTNER_ACCOUNT_ID:role/PartnerReadOnlyRole"} specifies the IAM role from the partner's AWS account that is allowed access. Replace PARTNER_ACCOUNT_ID with the actual account ID of the partner and PartnerReadOnlyRole with the role name designated for this access.
• "Action": [ "dynamodb:GetItem", "dynamodb:Scan" ] lists the DynamoDB actions that theprincipal is allowed to perform. In this case, the partner account can only read data from the table using GetItem and Scan.
• "Resource": "arn:aws:dynamodb:YOUR_AWS_REGION:YOUR_ACCOUNT_ID:table/yourSharedTable" specifies the ARN of the DynamoDB table you are sharing. Replace YOUR_AWS_REGION with your table's AWS region, YOUR_ACCOUNT_ID with your AWS account ID, and YourSharedTable with the name of the DynamoDB table.

Integration With IAM Tools

Resource-based policies work seamlessly with IAM Access Analyzer for identifying unintended cross-account permissions and with Block Public Access (BPA) features to prevent unwanted public exposure of DynamoDB resources.

Integrating DynamoDB with IAM (Identity and Access Management) tools such as IAM Access Analyzer can significantly enhance the security and compliance of your DynamoDB resources. This integration helps identify and remediate unintended permissions, ensuring that your DynamoDB tables are accessible only by authorized entities under strict conditions. An access control policy example for such integration involves specifying conditions that limit access based on IAM policy evaluations or external findings.

Below is an example of the DynamoDB Table Access Control Policy with IAM Access Analyzer Integration. This policy example aims to restrict access to a DynamoDB table to only those IAM roles that have been verified and flagged as compliant by the IAM Access Analyzer. It leverages the "aws:RequestTag/tag-name" condition to check if the requestor's role has the specified tag, which could represent compliance status as determined by an external review or IAM Access Analyzer findings.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AccessBasedOnComplianceTag",
      "Effect": "Allow",
      "Principal": "*",
      "Action": [
        "dynamodb:PutItem",
        "dynamodb:GetItem",
        "dynamodb:UpdateItem",
        "dynamodb:DeleteItem",
        "dynamodb:Query",
        "dynamodb:Scan"
      ],
      "Resource": "arn:aws:dynamodb:YOUR_AWS_REGION:YOUR_ACCOUNT_ID:table/You",
      "Condition": {
        "StringEquals": {
          "aws:RequestTag/ComplianceStatus": "Verified"
        }
      }
    }
  ]
}

In the above policy: 

• "Sid": "AccessBasedOnComplianceTag" gives a unique identifier to the statement for ease of management.
• "Effect": "Allow" specifies that the statement permits actions if conditions are met.
• "Principal": "*" indicates that the policy can apply to any IAM entity (user or role), but actual access is still strictly controlled by the condition block.
• "Action": [...] lists the DynamoDB actions that are permitted if the conditions are satisfied. These include common operations such as GetItem, PutItem, UpdateItem, etc.
• "Resource": "arn:aws:dynamodb:YOUR_AWS_REGION:YOUR_ACCOUNT_ID:table/yourSecureTable" should be replaced with the ARN of your DynamoDB table.
• "Condition": {"StringEquals": {"aws:RequestTag/ComplianceStatus": "Verified"}} restricts access to only those requests tagged with a ComplianceStatus of Verified. This tag could be applied to IAM roles following a compliance audit or an automated review process using the IAM Access Analyzer.

Implementing Resource-Based Policies

Implementing resource-based policies involves specifying actions and permissions through a policy document that outlines the allowed or denied actions for different principals. AWS offers multiple avenues for policy creation, including the AWS Management Console, AWS CLI, AWS SDKs, and AWS CloudFormation, catering to a wide range of operational preferences and requirements.

Below is an example of Creating a Resource-Based Policy:

Consider a scenario where you need to grant an external AWS account's IAM role read-only access to a DynamoDB table. The policy might look something like this:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::ACCOUNT_ID:role/ExternalRole"
      },
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:BatchGetItem"
      ],
      "Resource": "arn:aws:dynamodb:REGION:ACCOUNT_ID:table/TableName"
    }
  ]
}

This policy specifies that the external role is allowed to perform GetItem and BatchGetItem actions on the specified DynamoDB table.

Best Practices and Common Use Cases

When deploying resource-based policies, it's beneficial to adhere to best practices such as:

• Regularly auditing your policies with tools like IAM Access Analyzer.
• Using BPA to ensure your DynamoDB resources are not unintentionally exposed to the public internet.
• Implementing policies that allow DynamoDB functions to access only the necessary tables and actions, thus embracing the principle of least privilege.

Common use cases include allowing Lambda functions to access specific tables, sharing DynamoDB streams across accounts for real-time data processing, and restricting access to sensitive data to specific roles or users.

Conclusion

The introduction of resource-based policies for Amazon DynamoDB represents a significant leap forward in access control, offering a flexible and secure mechanism to manage permissions for DynamoDB resources. By leveraging these policies, developers, and administrators can achieve a higher level of security, granular control, and operational efficiency in their cloud environments. For further exploration and advanced capabilities, the official DynamoDB documentation provides extensive information and guidance.