A simple lambda function to subscribe Firebase Cloud Messaging clients to a set of topics. Used to furnish web push notification support in the CTFd web UI during SunshineCTF 2019.

fcm_subscriber_lambda.py

from __future__ import print_function
import urllib2
import urllib
import json

CONFIG = {
    'CLIENT_API_KEY': "",
    'SERVER_API_KEY': "",
    'DEFAULT_TOPICS': ["all", "some_topic_here"]
}

def handler(event, context):
    if 'token' not in event:
        return response({"message": "bad request"}, 400)
    
    if len(event['token']) == 0:
        return response({"message": "bad request"}, 400)
        
    else:
        nsuccesses = 0
        for topic in CONFIG['DEFAULT_TOPICS']:
            if subscribe_to_topic(event['token'], topic):
                print("Subscribed client " + event['token'] + "to topic: " + topic)
                nsuccesses = nsuccesses + 1
            else:
                print("ERROR subscribing client: " + event['token'] + "to topic: " + topic)
    
    return response({"message": "subscribed to " + str(nsuccesses) + " topics"}, 200)
            
            
def subscribe_to_topic(token, topic):
    head = {
        'User-agent': 'firebasecm-subscriber/1.0',
        'Content-Type': 'application/json',
        'Authorization': 'key=' + CONFIG['SERVER_API_KEY']
    }
    try:
        res = urllib2.urlopen(urllib2.Request('https://iid.googleapis.com/iid/v1/'+token+'/rel/topics/'+topic, data="", headers=head))
        if res.getcode() == 200:
            return True
            
        print("Firebase responded with HTTP error " + result.getcode() + " for client: " + token)
        
    except:
        print("Exception in call to Firebase for client: " + token)
        pass
    
    return False

def response(message, status_code):
    return {
        'isBase64Encoded': False,
        'statusCode': str(status_code),
        'body': json.dumps(message),
        'headers': {
            'Content-Type': 'application/json',
            'Access-Control-Allow-Origin': '*'
            },
        }