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https://github.com/rogersanctus/csrf_plus

A plug-based CSRF protection library for Elixir
https://github.com/rogersanctus/csrf_plus

csrf csrf-plus csrf-protection elixir library session store

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A plug-based CSRF protection library for Elixir

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# CsrfPlus

A plug-based CSRF (Cross-Site Request Forgery) protection library with accesses storing support.



Sometimes, you need more than a per-request CSRF tokens. This is why this Plug was created.

This plug supports storing tokens in any kind of storage system. And all you have to do is to

implement its `CsrfPlus.Store.Behaviour`. By doing so you will provide ways to put, get,

delete and other operations of accesses on that 'Store'.



## How it works?

When a request is made, this plug will check the request method type. Request methods intended

for reading data (GET, HEAD, OPTIONS) will be ignored, by default. The other requests will be

checked against the CSRF token stored in the connection session, in the `x-csrf-token` header

and the one stored in the configured Store. All those tokens must match to each other to be

considered valid. The first checkings are to ensure the tokens are given. Then, the token in

the session and the one in the Store are verified against each other. Later, the token on the

`x-csrf-token` header that is a signed version of the generated token with the configured secret

key will be verified against that secret key. If this verification succeeds, the returned verified

token will be checked against the token in the store. If this verification also succeeds, the

connection continues normally. Otherwise, if any of the checks fails, an exception will be raised

and used to build the error message and response status code.



## Usage

You can use CsrfPlus as a plug in your endpoint, router or in some plug pipeline. As this plug

uses connection session, it must be plugged after `Plug.Session` and `Plug.Conn.fetch_session/2`.

Also, this plug won't check requests origin. So, to have safer connections, use some CORS lib of

your choice before CsrfPlus. A good choice is the [Corsica](https://github.com/whatyouhide/corsica) project.



One of the princeples of this plug is to avoid the most to do "dark magic". As so, you must generate the token and its signed version,

provide an `:access_id` and normal token in the user session and, store both the token and the `access_id` in the

configured store and, finally, include the signed token in the `x-csrf-token` header. Usually, you do this step in

response to a GET method request, thinking of a JSON API. But you can also use this plug with heex template

pages/components. More on the examples section.



Anyway, we provide a helper function to get the tokens ready to be send in the response connection and stored

in the configured store. This function is `CsrfPlus.put_token/2`. You will still need to generate, at least,

the token and its signed version. And to generate the token and its signed version you can use `CsrfPlus.Token.generate/0` that will

give you back a new token tuple at every call, or you can use the `CsrfPlus.get_token_tuple/1` that uses the `conn` struct to try to reuse

the token in the connection session or generate a new one when it's not possible. The use of the later function is prefferable when using

_heex pages/components_.



When using CORS, remmember to add the `x-csrf-token` header to the allowed and exposed headers. Also enable/allow the

session credentials.



### The access

Tokens are stored as accesses and each access must have a unique id, the `access_id`. You can use `UUID.uuid4/0`

to generate a unique id.

This `access_id` must be the same as the token in the session and the one in the store. This is because CsrfPlus

will, when checking a request, retrieve the `access_id` from the session and then use it to load the token in the

store. A found token with the given access id will be used later to test the tokens from session and `x-csrf-token`

in the header.



### Examples



An example project that uses phoenix heex pages can be found [here](https://github.com/rogersanctus/csrf_phx_example).



At the moment, LiveViews are not supported as they have a kind of hard coded validation through `Plug.CSRFProtection`. So if you want to

use LiveViews you must follow the instructions at `Phoenix.LiveView` [docs](https://hexdocs.pm/phoenix_live_view/welcome.html).



For an usage example on JSON Apis, check [recaptcha](https://github.com/rogersanctus/recaptcha) **(backend)** and

[recaptcha-front](https://github.com/rogersanctus/recaptcha-front) **(frontend)**.



## How to install?

Simply, add `:csrf_plus` to you mix dependencies:



```elixir

# mix.exs

def deps do

  [

    {:plug, "~> 1.0"},

    {:csrf_plus, "~> 0.1"}

  ]

```



And then run `$ mix deps.get`.



## Setting it up



### Configs

`CsrfPlus` uses some configurations to get ready to run.



#### Token module

This module is responsible to generate and verify tokens.

You can create your own Token module, since you implement the `CsrfPlus.Token` behaviour.



This lib ships with a 'DefaultToken' module, that is used for that. But if you

want to set a custom Token module, use the config:



```elixir

# config/config.exs

config :csrf_plus, CsrfPlus.Token, token_mod: YourTokenModule

```



When using the 'DefaultToken', you can define the function to be used to generate tokens

or use the default one.

To config the token generator function add the following to your config:



```elixir

# config/config.exs

config :csrf_plus, CsrfPlus.Token, token_generation_fn: &your_function/0

```



Such a function must return a string (binary) that is the token itself. That

token must be unique.



Finally, if you want to use the 'DefaultToken' module, you should set the

`:secret_key` to be used to generate and verify tokens. That key must be at least 64

bytes long.



Config with:



```elixir

# config/config.exs

config :csrf_plus, CsrfPlus.Token, secret_key: "a_good_strong_random_secret"

```



> #### Please note {: .info}

> This `:secret_key` is only needed if you are using the 'DefaultToken'

> module.



#### Store module

In short, the store module is the module that implements the Store behaviour

and will keep your generated tokens ready to be checked against. If any served

token is not on the store, it is considered as invalid. For more information about

Stores look at the `CsrfPlu.Store.Behaviour` module.

To config the used store module, add the following:



```elixir

# config/config.exs

config :csrf_plus, CsrfPlus, store: YourStoreModule

```



There is no store set by default. But if you don't care about that, you can use

the builtin `CsrfPlus.Store.MemoryDb` store. As it names suggests, it will store

tokens (accesses, actually) in memory, using a `GenServer`.



If so, do:



```elixir

# config/config.exs

config :csrf_plus, CsrfPlus, store: CsrfPlus.Store.MemoryDb 

```



### Options

The `CsrfPlus` plug accepts some options at its initialization.

The following options are available:



  * `:csrf_key` - The key under the token is stored in the connection session. Defaults to: `"_csrf_key_"`



  * `:allowed_methods` - The requests methods that are ignored by the CSRF token validations.

By defaut: `["GET", "HEAD", "OPTIONS"]`.



  * `:raise_exception?` - If true, exceptions thrown by `CsrfPlus` will not be caught, but reraised. Set this to false

  to use the `ErrorMapper` and have response error and status code set. Defaults to `false`.



  * `:error_mapper` - The module to be used to map exceptions to response status codes and error messages. The connection is halted

  after the error is set.



    There is a default module for that: `CsrfPlus.ErrorMapper`. But you can set your own module

    since it implements the `CsrfPlus.ErrorMapper` behaviour. The `ErrorMapper` `map` function will only be used when

    the option `:raise_exception?` is set to false.



### Supervisor

As the CSRF validation of this lib uses a store of tokens, it's a good practice to

let them have a life span. And to keep the Store of tokens updated by this rule, we

use a store manager `CsrfPlus.Store.Manager` that will check all the tokens in the store

and flag the expired tokens as so. That Manager can be started directly, but to make it

easier to use with the default `CsrfPlus.Store.MemoryDb` store, we provide a `CsrfPlus.Supervisor` that

will not only start them, but also keep them up.



To start the Supervisor include the following entry in your application start function:



```elixir

# application.ex

def start(_type, _args) do

  #...



  children = [

    #...

    {CsrfPlus.Supervisor, opts}

    #...

  ]



  #...

end

```



Replace opts by the options you want to set for the `CsrfPlus.Store.Manager`.

For more information about the options look at the module documentation.