When trying to connect to a database using a connection string where the database is not specified, an Authentication failed (code 18) error is thrown by the MongoClient.connect() method.

This bug is not present on versions before 3.1.8 and only happens when the new url parser is used.

When specifying the admin database in the connection string, the error does not occur.

mongodb://username:password@127.0.0.1:27017  --> does not work

mongodb://username:password@127.0.0.1:27017/admin --> works