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Basic Github Information

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Basic Structure

The basic github environment structure of a big company is to own an enterprise which owns several organizations and each of them may contain several repositories and several teams.. Smaller companies may just own one organization and no enterprises.

From a user point of view a user can be a member of different enterprises and organizations. Within them the user may have different enterprise, organization and repository roles.

Moreover, a user may be part of different teams with different enterprise, organization or repository roles.

And finally repositories may have special protection mechanisms.

Privileges

Enterprise Roles

  • Enterprise owner: People with this role can manage administrators, manage organizations within the enterprise, manage enterprise settings, enforce policy across organizations. However, they cannot access organization settings or content unless they are made an organization owner or given direct access to an organization-owned repository
  • Enterprise members: Members of organizations owned by your enterprise are also automatically members of the enterprise.

Organization Roles

In an organisation users can have different roles:

  • Organization owners: Organization owners have complete administrative access to your organization. This role should be limited, but to no less than two people, in your organization.
  • Organization members: The default, non-administrative role for people in an organization is the organization member. By default, organization members have a number of permissions.
  • Billing managers: Billing managers are users who can manage the billing settings for your organization, such as payment information.
  • Security Managers: It's a role that organization owners can assign to any team in an organization. When applied, it gives every member of the team permissions to manage security alerts and settings across your organization, as well as read permissions for all repositories in the organization.
    • If your organization has a security team, you can use the security manager role to give members of the team the least access they need to the organization.
  • Github App managers: To allow additional users to manage GitHub Apps owned by an organization, an owner can grant them GitHub App manager permissions.
  • Outside collaborators: An outside collaborator is a person who has access to one or more organization repositories but is not explicitly a member of the organization.

You can compare the permissions of these roles in this table: https://docs.github.com/en/organizations/managing-peoples-access-to-your-organization-with-roles/roles-in-an-organization#permissions-for-organization-roles

Members Privileges

In https://github.com/organizations/\<org_name>/settings/member_privileges you can see the permissions users will have just for being part of the organisation.

The settings here configured will indicate the following permissions of members of the organisation:

  • Be admin, writer, reader or no permission over all the organisation repos.
  • If members can create private, internal or public repositories.
  • If forking of repositories is possible
  • If it's possible to invite outside collaborators
  • If public or private sites can be published
  • The permissions admins has over the repositories
  • If members can create new teams

Repository Roles

By default repository roles are created:

  • Read: Recommended for non-code contributors who want to view or discuss your project
  • Triage: Recommended for contributors who need to proactively manage issues and pull requests without write access
  • Write: Recommended for contributors who actively push to your project
  • Maintain: Recommended for project managers who need to manage the repository without access to sensitive or destructive actions
  • Admin: Recommended for people who need full access to the project, including sensitive and destructive actions like managing security or deleting a repository

You can compare the permissions of each role in this table https://docs.github.com/en/organizations/managing-access-to-your-organizations-repositories/repository-roles-for-an-organization#permissions-for-each-role

You can also create your own roles in https://github.com/organizations/\<org_name>/settings/roles

Teams

You can list the teams created in an organization in https://github.com/orgs/\<org_name>/teams. Note that to see the teams which are children of other teams you need to access each parent team.

Users

The users of an organization can be listed in https://github.com/orgs/\<org_name>/people.

In the information of each user you can see the teams the user is member of, and the repos the user has access to.

Github Authentication

Github offers different ways to authenticate to your account and perform actions on your behalf.

Web Access

Accessing github.com you can login using your username and password (and a 2FA potentially).

SSH Keys

You can configure your account with one or several public keys allowing the related private key to perform actions on your behalf. https://github.com/settings/keys

GPG Keys

You cannot impersonate the user with these keys but if you don't use it it might be possible that you get discover for sending commits without a signature. Learn more about vigilant mode here.

Personal Access Tokens

You can generate personal access token to give an application access to your account. When creating a personal access token the user needs to specify the permissions to token will have. https://github.com/settings/tokens

Oauth Applications

Oauth applications may ask you for permissions to access part of your github information or to impersonate you to perform some actions. A common example of this functionality is the login with github button you might find in some platforms.

Some security recommendations:

  • An OAuth App should always act as the authenticated GitHub user across all of GitHub (for example, when providing user notifications) and with access only to the specified scopes..
  • An OAuth App can be used as an identity provider by enabling a "Login with GitHub" for the authenticated user.
  • Don't build an OAuth App if you want your application to act on a single repository. With the repo OAuth scope, OAuth Apps can act on _all_** of the authenticated user's repositorie**s.
  • Don't build an OAuth App to act as an application for your team or company. OAuth Apps authenticate as a single user, so if one person creates an OAuth App for a company to use, and then they leave the company, no one else will have access to it.
  • More in here.

Github Applications

Github applications can ask for permissions to access your github information or impersonate you to perform specific actions over specific resources. In Github Apps you need to specify the repositories the app will have access to.

Some security recommendations:

  • A GitHub App should take actions independent of a user (unless the app is using a user-to-server token). To keep user-to-server access tokens more secure, you can use access tokens that will expire after 8 hours, and a refresh token that can be exchanged for a new access token. For more information, see "Refreshing user-to-server access tokens."
  • Make sure the GitHub App integrates with specific repositories.
  • The GitHub App should connect to a personal account or an organisation.
  • Don't expect the GitHub App to know and do everything a user can.
  • Don't use a GitHub App if you just need a "Login with GitHub" service. But a GitHub App can use a user identification flow to log users in and do other things.
  • Don't build a GitHub App if you only want to act as a GitHub user and do everything that user can do.
  • If you are using your app with GitHub Actions and want to modify workflow files, you must authenticate on behalf of the user with an OAuth token that includes the workflow scope. The user must have admin or write permission to the repository that contains the workflow file. For more information, see "Understanding scopes for OAuth apps."
  • More in here.

Github Actions

This isn't a way to authenticate in github, but a malicious Github Action could get unauthorised access to github and depending on the privileges given to the Action several different attacks could be done. See below for more information.

Git Actions

Git actions allows to automate the execution of code when an event happen. Usually the code executed is somehow related to the code of the repository (maybe build a docker container or check that the PR doesn't contain secrets).

Configuration

In https://github.com/organizations/\<org_name>/settings/actions it's possible to check the configuration of the github actions for the organization.

It's possible to disallow the use of github actions completely, allow all github actions, or just allow certain actions.

It's also possible to configure who needs approval to run a Github Action and the permissions of the GITHUB_TOKEN of a Github Action when it's run.

Git Secrets

Github Action usually need some kind of secrets to interact with github or third party applications. To avoid putting them in clear-text in the repo, github allow to put them as Secrets.

These secrets can be configured for the repo or for all the organization. Then, in order for the Action to be able to access the secret you need to declare it like:

steps:
  - name: Hello world action
    with: # Set the secret as an input
      super_secret: ${{ secrets.SuperSecret }}
    env: # Or as an environment variable
      super_secret: ${{ secrets.SuperSecret }}

Example using Bash 

steps:
  - shell: bash
    env:
      SUPER_SECRET: ${{ secrets.SuperSecret }}
    run: |
      example-command "$SUPER_SECRET"

{% hint style="warning" %} Secrets can only be accessed from the Github Actions that have them declared.

Once configured in the repo or the organizations users of github won't be able to access them again, they just will be able to change them. {% endhint %}

Therefore, the only way to steal github secrets is to be able to access the machine that is executing the Github Action (in that scenario you will be able to access only the secrets declared for the Action).

Git Environments

Github allows to create environments where you can save secrets. Then, you can give the github action access to the secrets inside the environment with something like:

jobs:
  deployment:
    runs-on: ubuntu-latest
    environment: env_name

You can configure an environment to be accessed by all branches (default), only protected branches or specify which branches can access it.
It can also set a number of required reviews before executing an action using an environment or wait some time before allowing deployments to proceed.

Git Action Runner

A Github Action can be executed inside the github environment or can be executed in a third party infrastructure configured by the user.

Several organizations will allow to run Github Actions in a third party infrastructure as it use to be cheaper.

You can list the self-hosted runners of an organization in https://github.com/organizations/\<org_name>/settings/actions/runners

The way to find which Github Actions are being executed in non-github infrastructure is to search for runs-on: self-hosted in the Github Action configuration yaml.

It's not possible to run a Github Action of an organization inside a self hosted box of a different organization because a unique token is generated for the Runner when configuring it to know where the runner belongs.

If the custom Github Runner is configured in a machine inside AWS or GCP for example, the Action could have access to the metadata endpoint and steal the token of the service account the machine is running with.

Git Action Compromise

If all actions (or a malicious action) are allowed a user could use a Github action that is malicious and will compromise the container where it's being executed.

{% hint style="danger" %} A malicious Github Action run could be abused by the attacker to:

  • Steal all the secrets the Action has access to
  • Move laterally if the Action is executed inside a third party infrastructure where the SA token used to run the machine can be accessed (probably via the metadata service)
  • Abuse the token used by the workflow to steal the code of the repo where the Action is executed or even modify it. {% endhint %}

Branch Protections

Branch protections are designed to not give complete control of a repository to the users. The goal is to put several protection methods before being able to write code inside some branch.

The branch protections of a repository can be found in https://github.com/\/<reponame>/settings/branches

{% hint style="info" %} It's not possible to set a branch protection at organization level. So all of them must be declared on each repo. {% endhint %}

Different protections can be applied to a branch (like to master):

  • You can require a PR before merging (so you cannot directly merge code over the branch). If this is select different other protections can be in place:
    • Require a number of approvals. It's very common to require 1 or 2 more people to approve your PR so a single user isn't capable of merge code directly.
    • Dismiss approvals when new commits are pushed. If not, a user may approve legit code and then the user could add malicious code and merge it.
    • Require reviews from Code Owners. At least 1 code owner of the repo needs to approve the PR (so "random" users cannot approve it)
    • Restrict who can dismiss pull request reviews. You can specify people or teams allowed to dismiss pull request reviews.
    • Allow specified actors to bypass pull request requirements. These users will be able to bypass previous restrictions.
  • Require status checks to pass before merging. Some checks needs to pass before being able to merge the commit (like a github action checking there isn't any cleartext secret).
  • Require conversation resolution before merging. All comments on the code needs to be resolved before the PR can be merged.
  • Require signed commits. The commits need to be signed.
  • Require linear history. Prevent merge commits from being pushed to matching branches.
  • Include administrators. If this isn't set, admins can bypass the restrictions.
  • Restrict who can push to matching branches. Restrict who can send a PR.

{% hint style="info" %} As you can see, even if you managed to obtain some credentials of a user, repos might be protected avoiding you to pushing code to master for example to compromise the CI/CD pipeline. {% endhint %}

References

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