github

Excellent question. This is a common security and organizational challenge when working with third-party tools and automations.

The short answer is no, it is not possible to directly authorize a GitHub App or OAuth App to access only specific directories within a repository.

GitHub's permission model operates at the repository level. When you grant an app access to a repository, you grant it permissions (like Read on code, or Read & write on issues) for the entire repository. There is no native feature to scope that access down to a file or directory path.

However, there are several effective workarounds and architectural patterns to achieve a similar result. The best choice depends on your specific use case (e.g., CI/CD, code analysis, a custom bot).

Workarounds and Best Practices

Here are the most common strategies, from simplest to most complex:

1. The Best Practice: Split the Repository

This is the cleanest and most secure solution if your architecture allows for it.

How it works: Move the directory you want the app to access into its own dedicated repository. Then, grant the app access only to that new, smaller repository.
Pros:
- Most Secure: The app genuinely has zero access to your other code. This follows the principle of least privilege perfectly.
- Clear Separation: Enforces a clear boundary between different parts of your project.
Cons:
- Major Refactoring: Can be a significant architectural change, potentially breaking build processes, Git history, and developer workflows.
- Not Always Feasible: Impractical for tightly coupled codebases (e.g., a frontend app in /frontend and its backend in /backend within a monorepo).

2. The CI/CD Approach: Use GitHub Actions with `paths` Filters

This is the most common solution for automation and deployment workflows. You limit when the action runs, not what it can access.

How it works: You grant the GitHub Actions workflow permission to the entire repository, but you configure the workflow to only trigger when changes are made within specific directories.

Example workflow.yml:

name: Deploy Website

on:
  push:
    branches:
      - main
    paths:
      - 'apps/my-website/**' # Only run this workflow for changes in this directory
      - '.github/workflows/deploy-website.yml' # Also run if the workflow file itself changes

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Build and Deploy Website
        # Your deployment script only works with files from 'apps/my-website'
        run: |
          cd apps/my-website
          npm install
          npm run build
          # ... deploy commands ...

Pros:
- Easy to Implement: No need to restructure your repository.
- Effective for CI/CD: Perfect for triggering deployments or tests for a specific part of a monorepo.
Cons:
- Security Caveat: The workflow's token (secrets.GITHUB_TOKEN) still has access to the entire repository. If a malicious dependency in your build script steals the token, it can access the whole repo, not just the specified directory. The limitation is on the trigger, not the access.

3. The Advanced Approach: A Custom "Broker" Service

This pattern provides the ultimate control but requires significant development effort.

How it works: You create your own small application (e.g., a serverless function) that acts as a secure intermediary.
1. Your custom service is granted full access to the GitHub repository.
2. The third-party app is given credentials to talk to your service, not directly to GitHub.
3. When the third-party app needs to read a file, it makes a request to your service (e.g., GET /api/files/my-directory/file.js).
4. Your service validates that the requested path is within the allowed directory and then uses its powerful GitHub token to fetch the file from the repo and return it to the app.
Pros:
- Granular Control: You can implement any logic you want (e.g., allow reads but not writes, only allow access to .js files, etc.).
- Maximum Security for the Third-Party App: The app never gets a token that can access your GitHub repo directly.
Cons:
- High Complexity: You have to build, host, and maintain this middle-man service.
- Shifts Responsibility: You are now responsible for the security of your broker service and its powerful GitHub token.

4. The Specific Use Case: Deploy Keys

If your goal is to give a server (like a production VM) git clone access for deployment, a Deploy Key is a good option.

How it works: A Deploy Key is an SSH key that you add to a single repository's settings. It grants read-only (or optionally, read-write) access to that one repo.
Pros:
- Simple for git operations: Excellent for pulling code onto a server.
- Scoped to one repository.
Cons:
- Still repository-scoped: It does not limit access to a directory.
- Not for API access: It's for git protocol access (clone/pull/push), not for interacting with the GitHub API (e.g., managing issues or pull requests).

Summary Table

Method	How it Works	Best For	Security
Split Repository	Move directory to a new repo and grant access to it.	When security is paramount and architecture allows.	Very High
GitHub Actions `paths`	Trigger workflows only on changes to a specific path.	CI/CD, tests, and deployments in a monorepo.	Medium (Token has full access)
Custom Broker Service	Build a middle-man app to proxy and validate requests.	Highly sensitive environments needing custom logic.	High (If built correctly)
Deploy Keys	An SSH key granting `git clone` access to one repo.	Giving a deployment server pull/push access.	High (For its specific purpose)

Recommendation

For CI/CD in a monorepo, use GitHub Actions with paths filters. It's the standard, industry-accepted practice.
For maximum security with a third-party app, the best but hardest path is to split the repository.
If you can't split the repo and the security risk of the Actions token is unacceptable, you must consider building a custom broker service.

mgreenly/gemini.md