@aws/app-framework-for-github-apps-on-aws

Credential Manager

Credential Manager is part of the framework-for-github-app-on-aws (The framework). It provides a secure and scalable solution for managing GitHub App credentials on AWS, with minimal custom code.

Overview

The Genet Credential Manager is a nested CDK stack and Level 3 (L3) construct that helps you manage GitHub App credentials on AWS. It securely stores your app's private key, creates short-lived App Tokens using AWS KMS, and retrieves Installation Access Tokens. This removes the need to write custom authentication logic or directly manage sensitive keys. The component integrates with your CDK application and exposes well-defined APIs and permission grants for secure credential access. By placing this functionality behind a dedicated API, you not only simplify implementation but also gain an additional layer of fine-grained access control via IAM policies

The Credential Manager simplifies and secures GitHub App credential handling by providing:

• Secure storage of GitHub App private signing keys using AWS KMS
• APIs for generating GitHub App and Installation Access Tokens
• Minimal logic required in your business application

Why Credential Manager?

GitHub Apps require a long-lived App Signing Key, which is used to generate short-lived App Tokens. These App Tokens are then used to request Installation Access Tokens that authorize API access to GitHub resources. While this flow is secure and scalable by design, it introduces operational and security risks. App Signing Keys are sensitive, and if stored improperly, they are vulnerable to accidental leaks or misuse. Because these keys can be used to generate unlimited App Tokens, any compromise could grant unauthorized access across all installations of the App.

The Credential Manager addresses these challenges by storing the signing key securely in AWS KMS-managed Hardware Security Modules (HSMs). It uses KMS signing to generate JWT-based App Tokens, so the key never leaves the secure boundary. The system exposes IAM-authenticated Lambda Function URLs that allow trusted components to retrieve App and Installation Access Tokens without handling or accessing the key directly. This design eliminates the need to embed key logic in your business code, reduces the attack surface, and enforces least privilege through fine-grained IAM controls.

Usage

Prerequisite

Before using Credential Manager in your application, you must complete the following setup steps:

1. Deploy the Credential Manager CDK stack This provisions all required infrastructure including DynamoDB tables, Lambda functions.

2. Import your GitHub App's private key into AWS KMS Use the app-framework-ops-tool to securely import your GitHub App private key into KMS.

See the README.md for step-by-step instructions on importing your GitHub App key.

Stack Integration

To use the Credential Manager in your CDK application, simply add the CredentialManager L3 construct to your stack. This provision all the necessary infrastructure, including DynamoDB tables, Lambda functions, and KMS keys. Use the provided methods to allow your Lambda functions or other IAM principals to generate tokens. These tokens are obtained by invoking the generated Function URLs using Smithy clients in your business logic.

import { CredentialManager } from '@aws/app-framework-for-github-apps-on-aws';

class MyStack extends Stack {
    constructor(scope: App, id: string) {
        super(scope, id);
        // Some component of your infrastructure that needs to get GitHub credentials.
        const examplePrincipal: IGrantable = someLambdaFunction;

        // Create the credential manager
        const credManager = new CredentialManager(this, 'CredentialManager', {});

        // You retrieve endpoints from credentialManager properties to pass into your Smithy client in your business logic.
        const appTokenEndpoint: string = credentialManager.appTokenEndpoint;
        const installationAccessTokenEndpoint: string = credentialManager.installationAccessTokenEndpoint;

        // Grant token generation permissions to other resources
        credManager.grantGetAppToken(examplePrincipal);
        credManager.grantGetInstallationAccessToken(examplePrincipal);
    }
}

Available Methods

grantGetAppToken

Grants permission to invoke the App token generation endpoint:

grantGetAppToken(grantee: IGrantable): void

grantGetInstallationAccessToken

Grants permission to invoke the installation access token endpoint:

grantGetInstallationAccessToken(grantee: IGrantable): void

Smithy Client

To interact with Credential Manager’s APIs, use the framework generated Smithy client. This client handles AWS SigV4 signing and request construction for both App Token and Installation Access Token APIs.

Initialize Client

import { AppFrameworkClient } from '@aws/app-framework-for-github-apps-on-aws-client';

const client = new AppFrameworkClient({
  endpoint: '<your deployed Lambda Function URL>',
  region: '<your AWS region>',
  credentials: '<your AWS credential provider>', 
  sha256: Sha256, // Sha256 hsing algorithm
});

Example: Get Installation Access Token

import { GetInstallationTokenCommand } from '@aws/app-framework-for-github-apps-on-aws-client';

const command = new GetInstallationTokenCommand({
  appId: '<your App ID>',
  nodeId: '<your App installation target node_id>',
});

const response = await client.send(command);
const token = response.installationToken;

Example: Get App Token

import { GetAppTokenCommand } from '@aws/app-framework-for-github-apps-on-aws-client';

const command = new GetAppTokenCommand({
  appId: '<your App ID>',
});

const response = await client.send(command);
const token = response.appToken;

What Resources Are Created by Credential Manager

DynamoDB Tables

App Table

Stores GitHub App IDs and their corresponding private key ARNs

• Schema:

  • Partition Key: AppId (NUMBER)

• Configuration:

  • Billing Mode: PAY_PER_REQUEST
  • Point-in-Time Recovery: Enabled
  • Removal Policy: RETAIN

Installation Table

Tracks GitHub App installations with node_id, installation_id, and app_id

• Schema:

  • Partition Key: AppId (NUMBER)
  • Sort Key: NodeId (STRING)

• Global Secondary Indexes:

  • NodeID:

    • Partition Key: NodeId (STRING)
    • Sort Key: AppId (NUMBER)

  • InstallationID:

    • Partition Key: InstallationId (NUMBER)
    • Sort Key: AppId (NUMBER)

• Configuration:

  • Billing Mode: PAY_PER_REQUEST
  • Point-in-Time Recovery: Enabled

Lambda Functions

GitHub App Token Generator

The App Token Generator is a Lambda function that generates short-lived JWTs used to authenticate the GitHub App itself. It exposes a Function URL with AWS IAM authentication and performs RSA signing operations through KMS. Access is restricted to IAM principals explicitly granted through grantGetAppToken.

• Permissions:

  • KMS:Sign for GitHub App private keys
  • DynamoDB Read access to App table

Installation Access Token Generator

The Installation Access Token Generator is a Lambda function that retrieves GitHub Installation Access Tokens for a specific installation of your GitHub App. It is exposed through a Function URL with IAM authentication, and access is controlled via grantGetInstallationAccessToken.

• Permissions:

  • KMS:Sign for GitHub App private keys
  • DynamoDB Read access to both tables

Scheduler

GitHub App Sync Scheduler

Scans the GitHub App installation metadata every 30 minutes and updates the Installation Table. This ensures the table stays in sync with active installations and can be used to track installation lifecycles reliably.

Security Considerations

Data Protection

Credential Manager stores your GitHub App signing key using AWS KMS. All signing operations are delegated to KMS, ensuring that the private key never leaves KMS-managed HSMs.

AWS KMS does not maintain the durability of imported key materials at the same level as key material that AWS KMS generates. In the unlikely event of certain region-wide failures that affect AWS KMS, your imported key material may need to be re-imported into a new KMS key. However, the impact in this use case should be low, because you can simply go to your GitHub App settings and generate a new private key, then re-import the new key into KMS to resume operation.

Metadata is stored in DynamoDB with Point-in-Time Recovery enabled, and the App table uses a RETAIN removal policy to prevent accidental data loss.

Access Control

Function URLs require AWS IAM authentication, and only explicitly granted IAM principals can invoke them. Access to KMS keys is restricted via tag-based policies to enforce fine-grained permissions.

Cost Impact

These AWS resources incur usage-based charges:

1. DynamoDB

   • Pay-per-request billing for both tables
   • Point-in-Time Recovery costs
   • Storage costs for table data

2. Lambda

   • Function invocation charges
   • Memory usage
   • Function URL requests

3. KMS

   • Key storage fees
   • Signing operation charges

Important Note:

1. The App table's RETAIN removal policy means it will persist even if the stack is deleted, potentially leading to ongoing costs.

2. When you rotate App signing key, we do not automatically schedule the old key for deletion. You will need to schedule the old key for deletion. Failure to do so can result in ongoing KMS storage charges for unused keys.

Resource Identification

The Credential Manager uses AWS resource tags for resource identification and access control instead of relying on static resource names. This approach provides better flexibility and control over resource management.

Resources Tags

Resource	Tag Key	Tag Value
Stack	FrameworkForGitHubAppOnAwsManaged	CredentialManager
App Table	CredentialManager	AppTable
Installation Table	CredentialManager	AppInstallationTable
Function URLs	CredentialManager	AppTokenEndpoint / InstallationAccessTokenEndpoint

Tag-Based Access Control

The framework uses tags for controlling access to KMS keys. KMS signing permissions are restricted to keys with specific tags:

    StringEquals: {
      'aws:ResourceTag/FrameworkForGitHubAppOnAwsManaged': 'true',
      'aws:ResourceTag/Status': 'Active'
    }