Enhancing GitHub OAuth Flow: Preserving User Permissions

Introduction

This article discusses a common issue in applications using GitHub OAuth for authentication: inadvertently overwriting a user's existing token with one that has fewer permissions. We'll explore how to ensure the application preserves the user's original, broader-scoped token when requesting narrower scopes during subsequent logins.

The Problem

Many applications use GitHub OAuth to access user repositories. The initial login might request broad permissions (e.g., repo scope) to allow access to both public and private repositories. Later, the application might redirect the user to GitHub OAuth again, perhaps from a dedicated login page, but this time with a more limited scope (e.g., public_repo). If not handled carefully, the callback from this second OAuth flow can overwrite the existing token, potentially causing the application to lose access to the user's private repositories.

The Solution

The core principle is to compare the scopes of the existing token and the newly requested token. If the existing token has a broader scope, it should be preserved. Here's a breakdown of the steps:

1. Check Existing Token Scope: Before replacing the user's token, determine the scope of the existing token.
2. Compare Scopes: Compare the scope of the existing token with the scope requested in the current OAuth flow. If the existing token's scope encompasses the requested scope, preserve the existing token.
3. Preserve Broader Scope: Only update the token if the newly granted scope is broader than the existing one or if there is no existing token.

Here's an illustrative example in C (although OAuth token handling is typically done in a higher-level language):

// Example function to compare OAuth scopes (illustrative)
bool isScopeSufficient(const char *existingScope, const char *requestedScope) {
  // In a real implementation, you would parse the scopes
  // and check if all permissions in requestedScope are also
  // present in existingScope.
  // This is a placeholder.
  if (strcmp(existingScope, "repo") == 0 && strcmp(requestedScope, "public_repo") == 0) {
    return true; // "repo" scope includes "public_repo"
  }
  return false;
}

// Example usage in the OAuth callback handler
void handleOAuthCallback(char *existingToken, char *newToken, char *requestedScope) {
  char *existingScope = getTokenScope(existingToken); // Function to retrieve the scope from the existing token

  if (existingToken == NULL || !isScopeSufficient(existingScope, requestedScope)) {
    // Update the token as the new token has broader permissions or no token existed before.
    updateUserToken(newToken); // Function to update the user's token in the database.
  } else {
    // Preserve the existing token as it has sufficient permissions.
    printf("Preserving existing token with scope: %s\n", existingScope);
  }
  free(existingScope);
}

Key Takeaways

• Prioritize User Permissions: Avoid unintentionally reducing a user's access by carefully managing OAuth token scopes.
• Scope Comparison is Crucial: Implement a mechanism to compare the scopes of existing and new tokens.
• Preserve Broader Access: Always preserve the existing token if it grants broader access than the newly requested scope.

By implementing these strategies, applications can enhance the user experience and ensure consistent access to resources.