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https://github.com/baldowl/rack_csrf

Anti-CSRF Rack middleware
https://github.com/baldowl/rack_csrf

csrf-prevention rack rack-middleware ruby

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Anti-CSRF Rack middleware

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= Rack::Csrf {rdoc-image:https://circleci.com/gh/baldowl/rack_csrf.svg?style=svg}[https://circleci.com/gh/baldowl/rack_csrf] {rdoc-image:https://github.com/baldowl/rack_csrf/actions/workflows/ci.yml/badge.svg?branch=master}[https://github.com/baldowl/rack_csrf/actions?query=workflow%3ACI+branch%3Amaster] {rdoc-image:https://badge.fury.io/rb/rack_csrf.svg}[http://badge.fury.io/rb/rack_csrf]



This is just a small Rack middleware whose only goal is to lessen the hazards

posed by CSRF attacks by trying to ensure that all requests of particular

types come from the right client, not from a mischievous impersonator.



Rack::Csrf is not tailored to any particular web framework, so it can be used

with your preferred Rack-based framework.



== Usage



First of all, beyond Rack itself, there is only one prerequisite: you must set

up your rack with a session middleware, inserted anywhere before Rack::Csrf.



Every POST, PUT, DELETE and PATCH request will be searched for the

anti-forging token, randomly generated by Rack::Csrf and stored inside the

session. If there's a token and it matches with the stored one, then the

request is handed over to the next rack component; if not, Rack::Csrf

immediately replies with an empty response.



The anti-forging token can be passed as a request parameter or a header.



== Options



The following options allow you to tweak Rack::Csrf.



[:raise]

  Set it to true to change the handling of bad requests: instead of producing

  an empty response, Rack::Csrf will raise an exception of class

  Rack::Csrf::InvalidCsrfToken.



    use Rack::Csrf, :raise => true



  Default value: false.



[:check_only]

  By default, Rack::Csrf checks every POST, PUT, DELETE and PATCH request;

  passing an array of HTTP method/URL (regular expressions allowed) to this

  option you can change this behavior to *only* check the items on this list.



    use Rack::Csrf, :check_only => ['POST:/checking', 'PUT:/me_too',

      'DELETE:/cars/.*\.xml', 'PATCH:/this/.*/too']



  Please, note that the regular expressions are not escaped and it is your

  duty to write them correctly. Empty PATH_INFO (see Rack's spec for details)

  is treated as '/' for this check.



  Default value: empty.



[:skip]

  By default, Rack::Csrf checks every POST, PUT, DELETE and PATCH request;

  passing an array of HTTP method/URL (regular expressions allowed) to this

  option you can choose what to let pass unchecked:



    use Rack::Csrf, :skip => ['POST:/not_checking', 'PUT:/me_too',

      'DELETE:/cars/.*\.xml', 'PATCH:/this/.*/too']



  Please, note that the regular expressions are not escaped and it is your

  duty to write them correctly. Empty PATH_INFO (see Rack's spec for details)

  is treated as '/' for this check.



  Default value: empty.



[:skip_if]

  Sets a lambda/Proc executed on every request to determine if that request

  should be let pass untouched:



    use Rack::Csrf, :skip_if => lambda { |request|

      request.env.key?('HTTP_X_VERY_SPECIAL_HEADER')

    }



  Your code will receive a request object (see Rack's documentation for

  details); if it returns anything but nil or false, no further checking is

  performed and the request is let pass.



  This option is useful if a guarded resource can be accessed by clients who

  support CSRF token (e.g. browsers) and by ones who don't (e.g. API clients).



  Don't try to check the CSRF token: it could exist or not at this stage and

  you should always let it alone.



  Default value: empty.



[:field]

  Default field name (see below) is _csrf; you can adapt it to

  specific needs.



    use Rack::Csrf, :field => '_my_own_csrf_field'



  Default value: _csrf



[:key]

  The key used to store/retrieve the token from the Rack session; you can

  adapt it to specific needs.



    use Rack::Csrf, :key => 'my.own_session.key'



  Default value: csrf.token



[:header]

  Default header name (see below) is X_CSRF_TOKEN; you can adapt it

  to specific needs.



    use Rack::Csrf, :header => 'MY_CSRF_TOKEN_HEADER'



  This is useful if we want to configure our application to send the CSRF

  token in all of our AJAX requests via a header. We could implement something

  along the lines of the following:



    (function(jQuery) {

      /*

       * Set the CSRF token for each AJAX request, Rack::Csrf handle the rest.

       * Assumes your layout has a metatag with name of "_csrf" and you're

       * using the default Rack:Csrf header setup.

       */

      jQuery.ajaxSetup({

        beforeSend: function(xhr) {

          var token = jQuery('meta[name="_csrf"]').attr('content');

          xhr.setRequestHeader('X_CSRF_TOKEN', token);

        }

      });

    }(jQuery));



  Default value: X_CSRF_TOKEN



  Note that Rack will append "HTTP_" to this value.



[:check_also]

  By passing an array of uppercase strings to this option you can add them to

  the list of HTTP methods which "mark" requests that must be searched for the

  anti-forging token.



    use Rack::Csrf, :check_also => %w(WHATEVER YOU WANT EVEN GET)



  Default value: empty



The :browser_only option has been removed; you do not need to edit

any rackup file because Rack::Csrf simply ignores unknown options. Changes

introduced in Rack version 1.1.0 tightened the parsing of POST params, so

requests need to have the right Content-Type (or none at all); these

Content-Types are exactly those used also by browsers and so there is no use

for :browser_only anymore.



The ill devised :browser_only option could have been used to

"protect" an API, but I think it might be better to use a combination of

:skip and formatted URLs.



== Helpers



The following class methods try to ease the insertion of the anti-forging

token.



[Rack::Csrf.key (also Rack::Csrf.csrf_key)]

  Returns the name of the key used to store/retrieve the token from the Rack

  session.



  Despite this class method, you should never try to retrieve the token with

  code like env['rack.session'][Rack::Csrf.key]. See the `token`

  method below.



[Rack::Csrf.field (also Rack::Csrf.csrf_field)]

  Returns the name of the field that must be present in the request.



[Rack::Csrf.header (also Rack::Csrf.csrf_header)]

  Returns the name of the header that must be present in the request.



[Rack::Csrf.token(env) (also Rack::Csrf.csrf_token(env))]

  Given the request's environment, it generates a random token, stuffs it in

  the session and returns it to the caller or simply retrieves the already

  stored one.



[Rack::Csrf.tag(env) (also Rack::Csrf.csrf_tag(env))]

  Given the request's environment, it generates a small HTML fragment to

  insert the token in a standard form like an hidden input field with the

  right value already entered for you.



[Rack::Csrf.metatag(env, options = {}) (also Rack::Csrf.csrf_metatag(env, options = {}))]

  Given the request's environment, it generates a small HTML fragment to

  insert the token in a standard metatag within your layout's head with the

  right value already entered for you.



  options is an optional hash that can currently take a +name+

  setting, which will alter the metatag's name attribute.



  Default name: _csrf



== Working examples



In the +examples+ directory there are some small, working web applications

written with different Rack-based frameworks. They are named after the used

framework; see the various README files for other details.



== Supported Rubies and Racks



The gemspec shows the minimum Ruby and Rack versions, but Rack::Csrf is

tested only with the Rubies and Racks you can see in

.circleci/config.yml and/or .github/workflows/ci.yml. It

could work also with older versions, but I decided not to test it against

unsupported Rubies and Racks.



== Contributing



If you want to help:



* fork the project[https://github.com/baldowl/rack_csrf] on GitHub;

* work in a topic branch;

* add features/specs for your additions or bug fixes;

* write your additions/bug fixes;

* commit;

* send me a pull request for the topic branch.



If you have any issue, please post them on the {project's issue

list}[https://github.com/baldowl/rack_csrf] on GitHub.



== Warning! Warning! Warning!



I cannot stress enough that this middleware is not a bulletproof vest or a

panacea for the CSRF plague; it is just an *aid* and by using it you cannot

forgo responsibilities for keeping your application as safe as possible.



== Copyright



Copyright (c) 2009, 2010, 2011, 2012, 2014, 2016, 2022 Emanuele Vicentini. 

See LICENSE.rdoc for details.