Version: Celery 4.0.2 (from looking at the changes since then it seems there is no change addressing this issue here: https://github.com/celery/celery/commits/master/celery/backends/mongodb.py )

(Edit) Confirmed with the following versions as well:
amqp==2.2.2
billiard==3.5.0.3
celery==4.1.0
kombu==4.1.0
pymongo==3.6.0

Celery Report

software -> celery:4.0.2 (latentcall) kombu:4.0.2 py:2.7.8
billiard:3.5.0.3 py-amqp:2.2.1
platform -> system:Linux arch:64bit, ELF imp:CPython
loader -> celery.loaders.default.Loader
settings -> transport:amqp results:disabled

Steps to reproduce

Give Celery a Backend URL pointing to a MongoDB instance with authentication and username/password (user/pwd set on the Admin DB by default) in the format:

mongodb://user:pass@your-server/your_db?authSource=admin

(Please see http://api.mongodb.com/python/current/examples/authentication.html#default-database-and-authsource and http://api.mongodb.com/python/current/api/pymongo/mongo_client.html?highlight=authsource )

Expected behavior

Celery authenticates the user in the admin database (this is the same as passing --authenticationDatabase to the mongo client or the same url to MongoClient)

Actual behavior

Celery tries to authenticate the user on the your_db database (failing to authenticate)

Workaround (not recommended)

Change the db on the URL to /admin (this db shouldn't be used to store arbitrary data normally)