能否获取其他用户的ID Token？Google OAuth2 Web应用预添加用户需求

Great question—this is a super common pain point when building apps that rely on Google OAuth for auth but need to set up users before their first login. Let’s break down what you can and can’t do here:

Can you get another user’s ID Token upfront? #

Short answer: No, and you shouldn’t try. Google’s OAuth2 framework is built to protect user privacy and security. ID Tokens are generated only when a user actively initiates the login flow and grants your app access—there’s no API endpoint that lets you retrieve another user’s ID Token without their direct involvement. Attempting to work around this would violate Google’s terms of service and put user data at risk.

Practical Workarounds for Pre-Provisioning #

Luckily, there are solid approaches to solve this problem depending on your use case:

1. Use Google Workspace Admin SDK (For Enterprise/Workspace Environments) #

If your users are part of a Google Workspace domain, you’re in luck. The Google Workspace Admin Directory API lets you fetch user details (including their unique id field) for users in your domain before they log into your app. Here’s the key detail: this id field is exactly the same as the sub value in their ID Token.

To implement this:

• Enable the Admin Directory API in your Google Cloud Console.
• Grant your service account the necessary permissions (minimum: https://www.googleapis.com/auth/admin.directory.user.readonly).
• Use ScribeJava or direct API calls to fetch user records, extract the id field, and store it as the sub in your database for pre-provisioned users.

This works seamlessly because Workspace’s user ID is tied directly to the OAuth2 sub claim.

2. Pre-Provision with Email + Link on First Login #

If you’re dealing with consumer Google accounts (or can’t use the Admin SDK), use the user’s email as a temporary identifier:

• When pre-provisioning, store the user’s email address in your database along with any admin-set permissions/metadata. Add a flag like is_provisioned: false to indicate they haven’t logged in yet.
• When the user first logs in via Google OAuth2, extract the email and email_verified claims from their ID Token.
• Match this verified email to your pre-provisioned record, then update the record with the sub value from the ID Token and set is_provisioned: true.

Important note: Always check that email_verified is true before linking—this prevents someone from spoofing an email to access a pre-provisioned account.

Bonus: Custom Invitation Flow #

For extra control, send a personalized invitation email to pre-provisioned users. The invitation link can include a unique token tied to their pre-provisioned record. When the user clicks the link, redirect them to your Google OAuth2 login flow. After login, use the invitation token to match their sub to the pre-provisioned entry. This ensures only the intended user can claim the pre-provisioned account.

Key Security Notes #

• Never store sensitive data in pre-provisioned records until the user has verified their identity via login.
• For the Admin SDK approach, follow the principle of least privilege—only grant the minimal permissions your app needs.
• Always validate ID Tokens (using Google’s public keys) before trusting any claims like sub or email.

内容的提问来源于stack exchange，提问作者Sam Hallerman