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.

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.

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.

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

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.

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.

If you want to help:

• fork the project 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 on GitHub.

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 © 2009, 2010, 2011, 2012, 2014, 2016, 2022 Emanuele Vicentini. See LICENSE.rdoc for details.