workflow-application-token-action

This is a GitHub Action that can be used to get scoped limited access, expiring credentials for use inside GitHub Actions Workflows.

Why would you want to do this? Well the GITHUB_TOKEN whilst having an expiry, has some protections around creating events that prevent downstream GitHub Actions workflow from triggering. This prevents recursive loops from workflows, but there are a number of valid types of workflows that may require or desire triggering downstream GitHub Actions Workflows.

The existing way to work around this today is to use a Personal Access Token, but these tokens are tied to a user and generally are over priviledged for the tasks at hand, increasing the risk if they get exposed and are not time limited like the GITHUB_TOKEN.

This is where a GitHub Application access token can really help out. The benefits of GitHub Applications is that you can restrict/scope the access of the token considerably more than what can be achieved using a Personal Access Token. The access token from the GitHub Application is also time limited, expiring after an hour from being issued, providing some more protection against any leaking of credentials from a Workflow.

Usage

To use this action you first need a GitHub Application created so that you can request temporary credentials on behalf of the application inside your workflows.

Requirements:

A new or existing GitHub Application with the access scopes required
A private key for the GitHub Application
The GitHub Application installed on the repository that the GitHub Actions Workflow will execute from

Creating a GitHub Application

You will need to have a GitHub Application that is scoped with the necessary permissions for the token that you want to retrieve at runtime.

To create a GitHub Application you can follow the steps available at https://docs.github.com/en/developers/apps/creating-a-github-app

The important configuration details for the application are:

GitHub App name a human readable application name that is unique within GitHub.com
Description some details about your application and what you intend to use it for
Homepage URL needs to be set to something as long as it is a URL
Expire user authorization tokens should be checked so as to expire any tokens that are issued
Webhook Active checkbox should be unchecked
Repository permissions, Organization permissions and/or User permissions should be set to allow the access required for the token that will be issued
Where can this GitHub App be installed? should be scoped to your desired audience (the current account, or any account)

Once the application has been created you will be taken to the General settings page for the new application. The GitHub Application will be issued an App ID which you can see in the About section, take note of this for later use in the Actions workflow.

On the General settings page for the application, at the bottom there is a Private keys section that you can use to generate a private key that can be utilized to authenticate as the application. Generate a new private key and store the information for later use.

Note: the private keys can and should be rotated periodically to limit the risks of them being exposed in use.

Install the GitHub Application

Once you have the GitHub Application defined, you will need to install the application on the target organization or repository/ repositories that you want it to have access to. These will be any repositories that you want to gather information from or want the application to modify as per the scopes that were defined when the application was installed.

Note: The GitHub Application will need to be installed on the organization and or repository that you are executing the GitHub Actions workflow from, as the implementation requires this to be able to generate the access tokens.

Using the GitHub Action in a Workflow

To use the action in a workflow, it is recommended that you store the GitHub Application Private key in GitHub Secrets. This can be done at a repository or organization level (provided that the actions workflow has access to the secret).

When storing the Private key, you can store the raw PEM encoded certificate contents that the GitHub Application generates for you or Base64 encode it in the secret.

Parameters

application_id: The GitHub Application ID that you wil be getting the access token for
application_private_key: A private key generated for the GitHub Application so that you can authenticate (PEM format or base64 encoded)
permissions: The optional limited permissions to request, specifying this allows you to request a subset of the permissions for the underlying GitHub Application. Defaults to all permissions available to the GitHub Application when not specified. Must be provided in a comma separated list of token permissions e.g. issues:read, secrets:write, packages:read
organization: An optional organization name if the GitHub Application is installed at the Organization level (instead of the repository).
github_api_base_url: An optional URL to the GitHub API, this will be read and loaded from the runner environment by default, but you might be bridging access to a secondary GHES instance or from GHES to GHEC, you can utilize this to make sure the Octokit library is talking to the right GitHub instance.
https_proxy: An optional proxy to use for connecting with the GitHub instance. If the runner has HTTP_PROXY or HTTPS_PROXY specified as environment variables it will attempt to use those if this parameter is not specified.
revoke_token: An optional boolean true or false value to revoke the access token as part of the post job steps in the actions workflow. To preserve backwards compatibility on this action, it defaults to false.

Examples

Get a token with all the permissions of the GitHub Application:

jobs:
  get-temp-token:
    runs-on: ubuntu-latest

    steps:
      - name: Get Token
        id: get_workflow_token
        uses: peter-murray/workflow-application-token-action@v4
        with:
          application_id: ${{ secrets.APPLICATION_ID }}
          application_private_key: ${{ secrets.APPLICATION_PRIVATE_KEY }}

      - name: Use Application Token to create a release
        uses: actions/create-release@v1
        env:
          GITHUB_TOKEN: ${{ steps.get_workflow_token.outputs.token }}
        with:
          ....

Get a token with a limited subset of the permissions of the Github Application, in this case just the actions:write permission:

jobs:
  get-temp-token:
    runs-on: ubuntu-latest

    steps:
      - name: Get Token
        id: get_workflow_token
        uses: peter-murray/workflow-application-token-action@v4
        with:
          application_id: ${{ secrets.APPLICATION_ID }}
          application_private_key: ${{ secrets.APPLICATION_PRIVATE_KEY }}
          permissions: "actions:write"

      - name: Use Application Token to create a release
        uses: actions/create-release@v1
        env:
          GITHUB_TOKEN: ${{ steps.get_workflow_token.outputs.token }}
        with:
          ....

Get a token with all the permissions of the Github Application that is installed on an organization:

jobs:
  get-temp-token:
    runs-on: ubuntu-latest

    steps:
      - name: Get Token
        id: get_workflow_token
        uses: peter-murray/workflow-application-token-action@v4
        with:
          application_id: ${{ secrets.APPLICATION_ID }}
          application_private_key: ${{ secrets.APPLICATION_PRIVATE_KEY }}
          organization: octodemo

      - name: Use Application Token to create a release
        uses: actions/create-release@v1
        env:
          GITHUB_TOKEN: ${{ steps.get_workflow_token.outputs.token }}
        with:
          ....

Proxy

You can specify a proxy server directory using the https_proxy parameter in your with settings, or by falling back to using any environment variables used to provide a proxy reference; HTTP_PROXY or HTTPS_PROXY (or lowercase variants e.g. http_proxy). If defined, the request will use the proxy to route the connection to the GitHub instance.

jobs:
  get-temp-token:
    runs-on: ubuntu-latest

    steps:
      - name: Get Token
        id: get_workflow_token
        uses: peter-murray/workflow-application-token-action@v4
        with:
          application_id: ${{ secrets.APPLICATION_ID }}
          application_private_key: ${{ secrets.APPLICATION_PRIVATE_KEY }}
          organization: octodemo
          https_proxy: http://my-squid-proxy:3128
          ....

In version 2.1.0 of this action support has been added for no_proxy environment variables that might be present in the workflows or actions runner.

If the https_proxy input variable is specified for the action, then any no_proxy setting will be ignored as the proxy in this case has been explicitly set and this action views that as being entirely intentional.

If on the other hand the proxy server is being detected using environment variables, http_proxy, HTTP_PROXY, https_proxy, HTTPS_PROXY and no_proxy is present then it will be parsed for hostname matches as to whether or not to use the proxy when access the GitHub API.

The format that is supported for no_proxy environment variable is a comma separated list of host names, e.g. api.github.com,www.google.com of when to not use the proxy server.

Access Token revocation

To provide additional options for security around the access token and waiting on it to expire, you can leverage the revoke_token input set to true so that at the end of the job run, a post actions step will revoke the access token, invalidating it so that is is immediately invalid and cannot be used.

References

https://docs.github.com/en/developers/apps/authenticating-with-github-apps#authenticating-as-an-installation

License

asml-actions-validation/workflow-application-token-action

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