Verify Bearer Tokens

A Bearer Token is set in the Authorization header of every In-App Action HTTP Request. For example:

POST /approve?expenseId=abc123 HTTP/1.1
Host: your-domain.com
Authorization: Bearer AbCdEf123456
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/1.0 (KHTML, like Gecko; Gmail Actions)

confirmed=Approved

The string "AbCdEf123456" in the example above is the bearer authorization token. This is a cryptographic token produced by Google. All bearer tokens sent with actions have the azp (authorized party) field as gmail@system.gserviceaccount.com, with the audience field specifying the sender domain as a URL of the form https://. For example, if the email is from noreply@example.com, the audience is https://example.com.

If using bearer tokens, verify that the request is coming from Google and is intended for the the sender domain. If the token doesn't verify, the service should respond to the request with an HTTP response code 401 (Unauthorized).

Bearer Tokens are part of the OAuth V2 standard and widely adopted by Google APIs.

Verifying Bearer Tokens

Services are encouraged to use the open source Google API Client library to verify Bearer tokens:

Java

import java.io.IOException;
import java.security.GeneralSecurityException;
import java.util.Collections;

import com.google.api.client.googleapis.auth.oauth2.GoogleIdToken;
import com.google.api.client.googleapis.auth.oauth2.GoogleIdTokenVerifier;
import com.google.api.client.http.apache.ApacheHttpTransport;
import com.google.api.client.json.jackson2.JacksonFactory;

public class TokenVerifier {
    // Bearer Tokens from Gmail Actions will always be issued to this authorized party.
    private static final String GMAIL_AUTHORIZED_PARTY = "gmail@system.gserviceaccount.com";

    // Intended audience of the token, based on the sender's domain
    private static final String AUDIENCE = "https://example.com";

    public static void main(String[] args) throws GeneralSecurityException, IOException {
        // Get this value from the request's Authorization HTTP header.
        // For example, for "Authorization: Bearer AbCdEf123456" use "AbCdEf123456"
        String bearerToken = "AbCdEf123456";

        GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(new ApacheHttpTransport(), new JacksonFactory())
                .setAudience(Collections.singletonList(AUDIENCE))
                .build();

        GoogleIdToken idToken = verifier.verify(bearerToken);
        if (idToken == null || !idToken.getPayload().getAuthorizedParty().equals(GMAIL_AUTHORIZED_PARTY)) {
            System.out.println("Invalid token");
            System.exit(-1);
        }

        // Token originates from Google and is targeted to a specific client.
        System.out.println("The token is valid");

        System.out.println("Token details:");
        System.out.println(idToken.getPayload().toPrettyString());
    }
}

Python

import sys

from oauth2client import client

# Bearer Tokens from Gmail Actions will always be issued to this authorized party.
GMAIL_AUTHORIZED_PARTY = 'gmail@system.gserviceaccount.com'

# Intended audience of the token, based on the sender's domain
AUDIENCE = 'https://example.com'

try:
  # Get this value from the request's Authorization HTTP header.
  # For example, for "Authorization: Bearer AbCdEf123456" use "AbCdEf123456"
  bearer_token = 'AbCdEf123456'

  # Verify valid token, signed by google.com, intended for a third party.
  token = client.verify_id_token(bearer_token, AUDIENCE)
  print('Token details: %s' % token)

  if token['azp'] != GMAIL_AUTHORIZED_PARTY:
    sys.exit('Invalid authorized party')
except:
  sys.exit('Invalid token')

# Token originates from Google and is targeted to a specific client.
print('The token is valid')