Setup for reproducing the issue

---------------------------------------- app to use

require 'sinatra'
require 'mongo'
require 'json'

@@conn= Mongo::Connection.new('192.168.0.192', 27017);
@@db1= @@conn.db('tweet')
@@db1.authenticate('admin', 'admin')

get '/' do
  puts @@db1
  @coll= @@db1.collection('tweets')
  @cursor= @coll.find({}, :fields => ['_id']).limit(2)
  @results= @cursor.to_a();
  JSON.dump(@results)
end

------------------------------------------ Data population script from console

use tweet
db.tweets.drop();

for(var i=0;i<200;i++){
  print(i)
  db.tweets.insert({account_id:new ObjectId(), avg_ctr:Math.random(), published:new Date() })
}

------------------------------------------ Steps to reproduce
1. Boot up mongod in auth mode
2. Run the console script to set up a database with some test data
3. Set up the user for the 'tweet' db using

use tweet
db.addUser('admin', 'admin')

4. start the application

ruby server.rb (or where you stored the file)

5. Hit the url with your browser or curl a couple of times

http://localhost:4567

6. Go to the mongo console and do

db.serverStatus()['cursors']

You will see the number of open cursor increase and hang around until they time out. This does not seem to happen if you run against the server on localhost or without auth.