Phoenix Todos - Authorized Sockets

This post is written as a set of Literate Commits. The goal of this style is to show you how this program came together from beginning to end.

Each commit in the project is represented by a section of the article. Click each section's header to see the commit on Github, or check out the repository and follow along.

Written by Pete Corey on Nov 9, 2016.

Authenticated Sockets

Now that we’ve implemented the bulk of the unauthenticated functionality in our application, we need to turn our attention to authenticated functionality.

To do that, we’ll need to authenticate the channel we’re using to communicate with the client. We can do this with a custom connect function that verified the user’s provided guardian_token:

def connect(%{"guardian_token" => jwt}, socket) do
  case sign_in(socket, jwt) do
    {:ok, authed_socket, guardian_params} ->
      {:ok, authed_socket}
    _ ->
      {:ok, socket}

If the user is authenticated correctly, we’ll swap their socket out for an auth_socket, which can be used to access the current user’s object and claims.

If the user doesn’t provide a guardian_token, we’ll fall back to our old connect function:

  def connect(_params, socket) do
    {:ok, socket}

All of the old functionality will continue to work as expected.


... use Phoenix.Socket + import Guardian.Phoenix.Socket ... # performing token verification on connect. + def connect(%{"guardian_token" => jwt}, socket) do + case sign_in(socket, jwt) do + {:ok, authed_socket, _guardian_params} -> + {:ok, authed_socket} + _ -> + {:ok, socket} + end + end + def connect(_params, socket) do

Guardian Token

Now that our socket connection is expecting a guardian_token parameter, we need to supply it in our connectSocket action.


... params: { - token: jwt + guardian_token: jwt }

Connect Socket Thunk

Because our entire socket connection will be authenticated or unauthenticated, we need to prepare ourselves to re-establish the connection every time we log in/out.

To start, we’ll need to clear out our local set of lists every time we connect to the socket.

We also need to trigger a call to our joinListsChannel thunk every time we connect.


... export function connectSocket(jwt) { - let socket = new Socket("/socket", { - params: { - guardian_token: jwt - } - }); - socket.connect(); - return { type: CONNECT_SOCKET, socket }; + return (dispatch, getState) => { + let socket = new Socket("/socket", { + params: { + guardian_token: jwt + } + }); + socket.connect(); + dispatch({ type: CONNECT_SOCKET, socket }); + dispatch(joinListsChannel("lists.public")); + }; }


... store.dispatch(connectSocket(store.getState().jwt)); -store.dispatch(joinListsChannel("lists.public"));


... case CONNECT_SOCKET: - return Object.assign({}, state, { socket: action.socket }); + return Object.assign({}, state, { socket: action.socket, lists: [] }); case JOIN_LISTS_CHANNEL_SUCCESS:


Now we’ll reconnect to our socket every time a user signs in, signs up, or signs out. This ensures that the socket connection is always properly authenticated.

These changes also introduced a few small bugs which we quickly fixed.


... dispatch(signUpSuccess(res.user, res.jwt)); + dispatch(connectSocket(res.jwt)); return true; ... dispatch(signOutSuccess()); + dispatch(connectSocket(res.jwt)); return true; ... dispatch(signInSuccess(res.user, res.jwt)); + dispatch(connectSocket(res.jwt)); return true;


... import { signOut, createList } from "../actions"; +import _ from "lodash"; ... const list = _.find(this.props.lists, list => ==; - if (list.user_id) { + if (list && list.user_id) { const publicList = _.find(this.props.list, list => !list.user_id);

Fetching All Lists

To make things simpler, and to show off a feature of Phoenix, I’ve decided to merge the "lists.public" and "lists.private" publications into a single channel: "lists".

This channel will return all lists accessible by the current user, based on their authenticated socket.

We replaced the List.public function with List.all, which takes in a user_id. When user_id is nil, we return all public lists, as before. However, when user_id isn’t nil, we return all lists owned by that user (^user_id == list.user_id), and all public lists.


... - test "public" do + test "all" do user = User.changeset(%User{}, %{ ... }) - Repo.insert!(%List{ + |> Repo.preload(:todos) + private = Repo.insert!(%List{ name: "private", ... }) + |> Repo.preload(:todos) - lists = List |> List.public |> Repo.all + public_lists = List |> List.all(nil) |> Repo.all |> Repo.preload(:todos) + all_lists = List |> List.all( |> Repo.all |> Repo.preload(:todos) - assert lists == [public] + assert public_lists == [public] + assert all_lists == [public, private] end


... - def join("lists.public", _message, socket) do - lists = List |> List.public |> Repo.all + defp get_user_id(socket) do + case Guardian.Phoenix.Socket.current_resource(socket) do + user -> + + _ -> + nil + end + end + + def join("lists", _message, socket) do + lists = List |> List.all(get_user_id(socket)) |> Repo.all {:ok, lists, socket}


... # channel "rooms:*", PhoenixTodos.RoomChannel - channel "lists.public", PhoenixTodos.ListChannel + channel "lists", PhoenixTodos.ListChannel


... - def public(query) do + def all(query, nil) do from list in query, ... + def all(query, user_id) do + from list in query, + where: ^user_id == list.user_id or is_nil(list.user_id), + order_by: list.inserted_at, + preload: [:todos] + end + def findByName(query, name) do


... dispatch({ type: CONNECT_SOCKET, socket }); - dispatch(joinListsChannel("lists.public")); + dispatch(joinListsChannel("lists")); };

Final Thoughts

In hashing out this authorization scheme, I’ve realized there are lots of problems with this approach. Splitting communication across both WebSockets and REST endpoints creates lots of confusion around a user’s authorization state.

In hindsight, it would have been better to do everything over WebSockets and forget the REST user and sessions endpoints altogether. I’ll be sure to write up my thoughts around the problems with this kind of authorization and how to to it better in the future.

Next week, we should be able to finish up all authenticated functionality and finish up the Meteor to Phoeix migration project!

NoSQL Injection in Phoenix Applications

Written by Pete Corey on Nov 7, 2016.

NoSQL injection is a class of application vulnerability where a malicious user can inject control structures into a query against a NoSQL database. MongoDB is the usual victim in these types of attacks, for reasons we’ll discuss towards the end of the article.

Coming most recently from a Meteor background, NoSQL injection is no stranger to me. It’s one of the most prevalent vulnerabilities I find during Meteor security assessments.

Interestingly, as I’ve been diving headfirst into Elixir and the Phoenix framework, I’ve been seeing the NoSQL injection monster raising its ugly head.

Let’s take a look at what NoSQL injection actually is, what it looks like in an Elixir/Phoenix application, and how it can be prevented.

What is NoSQL Injection

NoSQL injection is an interesting vulnerability that’s especially prevalent in systems built with MongoDB. NoSQL injection can occur when a user’s unvalidated, unsanitized input is inserted directly into a MongoDB query object.

To make things more real, let’s demonstrate NoSQL injection with a (slightly contrived) example.

Imagine you have a Phoenix channel that removes shopping cart “item” documents from a MongoDB collection whenever it receives an "empty_cart" event:

def handle_in("empty_cart", cart_id, socket) do
  |> Mongo.delete_many("items", %{"cart_id" => cart_id})
  {:noreply, socket}

This code is making the assumption that the "empty_cart" channel event will always be invoked with cart_id as a string. However, it’s important to realize that cart_id can be any JSON-serializable type.

What would happy if a malicious user passed in {$gte: ""} as a cart_id? Our resulting MongoDB query would look like this:

Mongo.delete_many("items", %{"cart_id" => %{"$gte" => ""}})

This query would remove every item document in the database.

Similar types of attacks can be used to fetch large amounts of unauthorized data from find and findOne queries.

Even more dangerously (and more contrived), let’s imagine we have a channel event handler that lets users search through their cart items for items matching a user-provided key/value pair:

def handle_in("find_items", %{"key" => key, "value" => value}, socket) do
  items = MongoPool
  |> Mongo.find("items", %{
       key => value,
       "user_id" => socket.assigns[:user]._id
  |> Enum.to_list
  {:reply, {:ok, items}, socket}

This seems relatively safe. We’re assuming that the user will pass in values like "foo"/"bar" for "key" and "value", respectively.

However, what would happen if a malicious user passed in "$where" and "d = new Date; do {c = new Date;} while (c - d < 10000);" as a "key"/"value" pair?

The resulting MongoDB query would look like this:

Mongo.find("items", %{
  "$where" => "d = new Date; do {c = new Date;} while (c - d < 10000);",
  "user_id" => socket.assigns[:user].id

By exploiting the $where operator in this way, the malicious user could peg the CPU of the server running the MongoDB instance at 100% for ten seconds per document in the collection, preventing any other queries from executing during that time.

This malicious elixir loop could easily be modified to run indefinitely, requiring you to either kill the query manually, or restart your database process.

How to Prevent It

Preventing this flavor of NoSQL injection is fairly straight-forward. You simply need to make assertions about the types of your user-provided data.

If you’re expecting cart_id to be a string, make sure it’s a string before working with it.

In Elixir, this type of type checking can be neatly accomplished with pattern matching. We can patch up our first example with a simple pattern match that checks the type of cart_id:

def handle_in("empty_cart", cart_id, socket) when is_binary(cart_id) do
  |> Mongo.delete_many("items", %{"cart_id" => cart_id})
  {:noreply, socket}

The when is_binary(cart_id) guard expression asserts that cart_id is a binary type (i.e., a string) before pattern matching on this instance of the handle_in function.

If a malicious user passed in %{"$gte" => ""} for an cart_id, this version of our "empty_cart" handler would not be evaluated, preventing the possibility of NoSQL injection.

Our "find_items" example is also susceptible to query objects being passed in as value, and would benefit from guard clauses.

However, the fundamental flaw with this example is that user input is being directly used to construct a root level MongoDB query.

A better version of our "find_items" channel event handler might look something like this:

def build_query("name", value), do: %{ "name" => value }
def build_query("category", value), do: %{ "category" => value }

def handle_in("find_items",
              %{"key" => key,
                "value" => value},
              socket) when is_binary(key) and is_binary(value)
  query = build_query(key, value)
  |> Map.put("user_id", socket.assigns[:user]._id
  items = MongoPool
  |> Mongo.find("items", query)
  |> Enum.to_list
  {:reply, {:ok, items}, socket}

By mapping between the provided key value and a list of known MongoDB query objects, we know that nothing can be injected into the root of our query.

Alternatively, we can continue to use the raw value of key to construct our query, but we can add a key in ["name", "category"] guard clause to our handle_in function to assert that the user is only searching over the "name" or "category" fields:

def handle_in("find_items",
              %{"key" => key,
                "value" => value},
              socket) when key in ["name", "category"] and is_binary(value)

By preventing malicious users from controlling the root level of our MongoDB query, we can prevent several types of nasty NoSQL injection vulnerabilities within our application.

That being said, the best way to prevent these kinds of injection attacks is to use a query builder, like Ecto.

Unfortunately, as we discussed last week, the Mongo.Ecto adapter is currently in a state of flux and does not play nicely with Ecto 1.1 or Ecto 2.0.

Picking on MongoDB

This type of NoSQL injection mostly applies to applications using MongoDB. This is because MongoDB has made the “interesting” design decision to intermix query control structures and query data in a single query object.

If a malicious user can inject data into this object, they can potentially inject query control structures as well. This is the fundamental idea behind NoSQL injection.

Looking at other NoSQL databases, it becomes apparent that MongoDB is alone in making this design decision.

Redis, for example, is a much simpler solution overall. Redis doesn’t mix data and control structures. The query type is specified up-front, almost always by the application, and unescapable data follows.

As another example, CouchDB lets developers build custom queries through “views”, but these views are written in advance and stored on the server. They can’t be modified at runtime, let alone modified by a malicious user.

There are already a host of compelling reasons not to use MongoDB. I would add MongoDB’s decision to intermix data and control structures to this ever growing list.

Final Thoughts

While MongoDB does have its short-comings, it’s important to realize that it’s still being used extensively in the Real World™. In fact, MongoDB is the most popular NoSQL database, standing heads and shoulders above its competition in usage statistics.

For this reason, it’s incredibly important to understand MongoDB-flavored NoSQL injection and how to prevent it in your applications.

For more information on NoSQL injection, check out the “NoSQL Injection in Modern Web Applications” presentation I gave at last year’s Crater Conference, and be sure to grab a copy of my “Five Minute Introduction to NoSQL Injection”.

How to Use MongoDB with Elixir

Written by Pete Corey on Oct 31, 2016.

Many of the application’s I’ve developed for myself and for clients over the recent years are intimately tied to MongoDB. This means that any new technology stack I experiment with need to be able to work well with this database.

Elixir is no exception to this rule.

Thankfully, Elixir and the Phoenix framework are database agnostic. They don’t require you to be tied to a single database, and even offer options for interfacing with a wide variety of databases.

Let’s dig into how we can use MongoDB in an Elixir application.

This article was written with version ~0.1 of the MongoDB driver in mind. For instructions on using version 0.2 of the MongoDB driver, see How to Use MongoDB with Elixir - Revisited.

Ecto Adapter

A very common way of interacting with a database in an Elixir application is to use the Ecto package.

Ecto acts as a repository layer around your database. It lets you write queries in a unified languages, and supports communicating with many types of databases through “adapters”.

The Mongo.Ecto package is the Ecto adapter for MongoDB. Unfortunately, Mongo.Ecto is currently in a state of flux.

Mongo.Ecto is currently incompatible with Ecto 1.1. On top of that, work to support Ecto 2.0 is very much a work in progress.

All of this is to say that integrating with MongoDB through Ecto is not currently an option if you’re looking for a low-friction, fully supported solution.

MongoDB Driver

Under the hood, the Mongo.Ecto adapter makes use of Eric Meadows-Jönsson’s MongoDB driver package.

While Mongo.Ecto is in a state of flux, the MongoDB driver package seems to be stable and functional.

Setting up the MongoDB driver in your Elixir application is a simple process. Get started by following the documentation on GitHub. Once you’ve defined your MongoPool module, you can start the process and make your queries:

{:ok, _} = MongoPool.start_link(database: "test")

|> Mongo.find("collection", %{ "foo" => "bar" })
|> Enum.to_list

Database options can be passed into the connection pool when you start_link. For example, to connect to a "meteor" database on localhost:3001, you could initiate your connection pool with these options:

{:ok, _} = MongoPool.start_link(database: "meteor", port: 3001)

You can find all available options in the Mongo.Connection module, or through iex:

iex -S mix
> h Mongo.Connection.start_link

Final Thoughts

It’s a shame that the Mongo.Ecto adapter isn’t in a stable state. While the MongoDB driver works beautifully, it would be nice to leverage the unified interface of Ecto when building applications.

When using the MongoDB driver directly, you need to concern yourself with tightly coupling your application to your persistence method. Ecto provides a nice layer of decoupling between the two.

Additionally, using the MongoDB driver directly opens yourself up to the possibility of being vulnerable to NoSQL injection attacks. We’ll drive into that topic next week.