# Verify the Google ID token on your server side When using Google Identity Services or OAuth 2.0 authorization code flow, Google returns the ID token using POST method to the redirect endpoint. Alternatively, the OIDC implicit flow uses a GET request. Consequently, your application is responsible to securely transmit these received credentials to your server.
GET This is the implicit flow, the ID token is returned within the URL fragment, which the client-side JavaScript must parse. Your application is responsible for implementing its own validation mechanisms to ensure the request's authenticity and prevent attacks like CSRF. ```console HTTP/1.1 302 Found Location: https://#access_token=&token_type=bearer&expires_in=&scope=&state= ``` POST The ID token is sent back as the `credential` field. When preparing to send the ID Token to the server the GIS library automatically adds the `g_csrf_token` to the header cookie and the request body. This is an example POST request: ``` POST /auth/token-verification HTTP/1.1 Host: example.com Content-Type: application/json;charset=UTF-8 Cookie: g_csrf_token= Origin: https://example.com Content-Length: { "credential": "", "g_csrf_token": "", "client_id": "" } ```
1. Validating the `g_csrf_token` to prevent Cross-Site Request Forgery (CSRF) attacks: - Extract the CSRF token value from the `g_csrf_token` cookie. - Extract the CSRF token value from the request body. The GIS library includes this token in the POST request body as a parameter, also named `g_csrf_token`. - Compare the two token values - If both the values are present and match completely, the request is considered legitimate and originated from your domain. - If the values are not present or does not match, the request has to be rejected by the server. This check ensures that the request was initiated from JavaScript running on your own domain, as only your domain can access the `g_csrf_token` cookie. 2. Verify the ID token. To verify that the token is valid, ensure that the following criteria are satisfied: - The ID token is properly signed by Google. Use Google's public keys (available in [JWK](https://www.googleapis.com/oauth2/v3/certs) or [PEM](https://www.googleapis.com/oauth2/v1/certs) format) to verify the token's signature. These keys are regularly rotated; examine the `Cache-Control` header in the response to determine when you should retrieve them again. - The value of `aud` in the ID token is equal to one of your app's client IDs. This check is necessary to prevent ID tokens issued to a malicious app being used to access data about the same user on your app's backend server. - The value of `iss` in the ID token is equal to `accounts.google.com` or `https://accounts.google.com`. - The expiry time (`exp`) of the ID token has not passed. - If you need to validate that the ID token represents a Google Workspace or Cloud organization account, you can check the `hd` claim, which indicates the hosted domain of the user. This must be used when restricting access to a resource to only members of certain domains. The absence of this claim indicates that the account does not belong to a Google hosted domain. Using the `email`, `email_verified` and `hd` fields, you can determine if Google hosts and is authoritative for an email address. In the cases where Google is authoritative, the user is known to be the legitimate account owner, and you may skip password or other challenge methods. Cases where Google is authoritative: - `email` has a `@gmail.com` suffix, this is a Gmail account. - `email_verified` is true and `hd` is set, this is a Google Workspace account.
Users may register for Google Accounts without using Gmail or Google Workspace. When `email` does not contain a `@gmail.com` suffix and `hd` is absent, Google is not authoritative and password or other challenge methods are recommended to verify the user. `email_verified` can also be true as Google initially verified the user when the Google account was created, however ownership of the third party email account may have since changed. Rather than writing your own code to perform these verification steps, we strongly recommend using a Google API client library for your platform, or a general-purpose JWT library. For development and debugging, you can call our `tokeninfo` validation endpoint. ### Using a Google API Client Library Using one of the [Google API Client Libraries](https://developers.google.com/api-client-library) (e.g. [Java](https://github.com/googleapis/google-auth-library-java), [Node.js](https://github.com/google/google-api-nodejs-client), [PHP](https://developers.google.com/api-client-library/php/start/get_started), [Python](https://google-auth.readthedocs.io/)) is the recommended way to validate Google ID tokens in a production environment. Java To validate an ID token in Java, use the [GoogleIdTokenVerifier](https://github.com/googleapis/google-api-java-client/blob/master/google-api-client/src/main/java/com/google/api/client/googleapis/auth/oauth2/GoogleIdTokenVerifier.java) object. For example: ```java import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken; import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken.Payload; import com.google.api.client.googleapis.auth.oauth2.GoogleIdTokenVerifier; ... GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(transport, jsonFactory) // Specify the WEB_CLIENT_ID of the app that accesses the backend: .setAudience(Collections.singletonList(WEB_CLIENT_ID)) // Or, if multiple clients access the backend: //.setAudience(Arrays.asList(WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3)) .build(); // (Receive idTokenString by HTTPS POST) GoogleIdToken idToken = verifier.verify(idTokenString); if (idToken != null) { Payload payload = idToken.getPayload(); // Print user identifier. This ID is unique to each Google Account, making it suitable for // use as a primary key during account lookup. Email is not a good choice because it can be // changed by the user. String userId = payload.getSubject(); System.out.println("User ID: " + userId); // Get profile information from payload String email = payload.getEmail(); boolean emailVerified = Boolean.valueOf(payload.getEmailVerified()); String name = (String) payload.get("name"); String pictureUrl = (String) payload.get("picture"); String locale = (String) payload.get("locale"); String familyName = (String) payload.get("family_name"); String givenName = (String) payload.get("given_name"); // Use or store profile information // ... } else { System.out.println("Invalid ID token."); } ``` The `GoogleIdTokenVerifier.verify()` method verifies the JWT signature, the `aud` claim, the `iss` claim, and the `exp` claim. If you need to validate that the ID token represents a Google Workspace or Cloud organization account, you can verify the `hd` claim by checking the domain name returned by the `Payload.getHostedDomain()` method. The domain of the `email` claim is insufficient to ensure that the account is managed by a domain or organization. Node.js To validate an ID token in Node.js, use the [Google Auth Library for Node.js](https://github.com/google/google-auth-library-nodejs). Install the library: ``` npm install google-auth-library --save ``` Then, call the `verifyIdToken()` function. For example:
```javascript const {OAuth2Client} = require('google-auth-library'); const client = new OAuth2Client(); async function verify() { const ticket = await client.verifyIdToken({ idToken: token, audience: WEB_CLIENT_ID, // Specify the WEB_CLIENT_ID of the app that accesses the backend // Or, if multiple clients access the backend: //[WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3] }); const payload = ticket.getPayload(); // This ID is unique to each Google Account, making it suitable for use as a primary key // during account lookup. Email is not a good choice because it can be changed by the user. const userid = payload['sub']; // If the request specified a Google Workspace domain: // const domain = payload['hd']; } verify().catch(console.error); ``` The `verifyIdToken` function verifies the JWT signature, the `aud` claim, the `exp` claim, and the `iss` claim. If you need to validate that the ID token represents a Google Workspace or Cloud organization account, you can check the `hd` claim, which indicates the hosted domain of the user. This must be used when restricting access to a resource to only members of certain domains. The absence of this claim indicates that the account does not belong to a Google hosted domain. PHP To validate an ID token in PHP, use the [Google API Client Library for PHP](https://github.com/google/google-api-php-client/). Install the library (for example, using Composer): ``` composer require google/apiclient ``` Then, call the `verifyIdToken()` function. For example:
```php require_once 'vendor/autoload.php'; // Get $id_token via HTTPS POST. $client = new Google_Client(['client_id' => $WEB_CLIENT_ID]); // Specify the WEB_CLIENT_ID of the app that accesses the backend $payload = $client->verifyIdToken($id_token); if ($payload) { // This ID is unique to each Google Account, making it suitable for use as a primary key // during account lookup. Email is not a good choice because it can be changed by the user. $userid = $payload['sub']; // If the request specified a Google Workspace domain //$domain = $payload['hd']; } else { // Invalid ID token } ``` The `verifyIdToken` function verifies the JWT signature, the `aud` claim, the `exp` claim, and the `iss` claim. If you need to validate that the ID token represents a Google Workspace or Cloud organization account, you can check the `hd` claim, which indicates the hosted domain of the user. This must be used when restricting access to a resource to only members of certain domains. The absence of this claim indicates that the account does not belong to a Google hosted domain. Python To validate an ID token in Python, use the [verify_oauth2_token](https://google-auth.readthedocs.io/en/latest/reference/google.oauth2.id_token.html#google.oauth2.id_token.verify_oauth2_token) function. For example: ```python from google.oauth2 import id_token from google.auth.transport import requests # (Receive token by HTTPS POST) # ... try: # Specify the WEB_CLIENT_ID of the app that accesses the backend: idinfo = id_token.verify_oauth2_token(token, requests.Request(), WEB_CLIENT_ID) # Or, if multiple clients access the backend server: # idinfo = id_token.verify_oauth2_token(token, requests.Request()) # if idinfo['aud'] not in [WEB_CLIENT_ID_1, WEB_CLIENT_ID_2, WEB_CLIENT_ID_3]: # raise ValueError('Could not verify audience.') # If the request specified a Google Workspace domain # if idinfo['hd'] != DOMAIN_NAME: # raise ValueError('Wrong domain name.') # ID token is valid. Get the user's Google Account ID from the decoded token. # This ID is unique to each Google Account, making it suitable for use as a primary key # during account lookup. Email is not a good choice because it can be changed by the user. userid = idinfo['sub'] except ValueError: # Invalid token pass ``` The `verify_oauth2_token` function verifies the JWT signature, the `aud` claim, and the `exp` claim. You must also verify the `hd` claim (if applicable) by examining the object that `verify_oauth2_token` returns. If multiple clients access the backend server, also manually verify the `aud` claim. > [!IMPORTANT] > **Key Point:** The ID token is returned in the `credential` field, instead of the `g_csrf_token` field. 3. Once the token's validity is confirmed, you can use the information in the [Google ID token](https://developers.google.com/identity/gsi/web/reference/js-reference#credential) to correlate the account status of your site: - **An unregistered user:** You can show a sign-up user interface (UI) that allows the user to provide additional profile information, if required. It also allows the user to silently create the new account and a logged-in user session. - **An existing account that already exists in your site:** You can show a web page that allows the end user to input their password and link the legacy account with their Google credentials. This confirms that the user has access to the existing account. - **A returning federated user:** You can silently sign the user in. > [!IMPORTANT] > **Key Point:** Only use [Google ID token](https://developers.google.com/identity/gsi/web/reference/js-reference#credential) **`sub`** field as identifier for the user as it is unique among all Google Accounts and never reused. You should store the **`sub`** field and associate it with the user in your account management system. While you can use the email address from the ID token to check if the user has a existing account, **don't** use email address as an identifier because a Google Account can have multiple email addresses at different points in time.