Plug v1.4.1 Plug.CSRFProtection View Source

Plug to protect from cross-site request forgery.

For this plug to work, it expects a session to have been previously fetched. It will then compare the plug stored in the session with the one sent by the request to determine the validity of the request. For an invalid request the action taken is based on the :with option.

The token may be sent by the request either via the params with key “_csrf_token” or a header with name “x-csrf-token”.

GET requests are not protected, as they should not have any side-effect or change your application state. JavaScript requests are an exception: by using a script tag, external websites can embed server-side generated JavaScript, which can leak information. For this reason, this plug also forbids any GET JavaScript request that is not XHR (or AJAX).

Note that it is recommended to enable CSRFProtection whenever a session is used, even for JSON requests. For example, Chrome had a bug that allowed POST requests to be triggered with arbitrary content-type, making JSON exploitable. More info: https://bugs.chromium.org/p/chromium/issues/detail?id=490015

Token generation

This plug won’t generate tokens automatically. Instead, tokens will be generated only when required by calling Plug.CSRFProtection.get_csrf_token/0. The token is then stored in the process dictionary to be set in the request.

One may wonder: why the process dictionary?

The CSRF token is usually generated inside forms which may be isolated from the connection. Storing them in the process dictionary allows them to be generated as a side-effect, becoming one of those rare situations where using the process dictionary is useful.

Options

  • :session_key - the name of the key in session to store the token under
  • :with - should be one of :exception or :clear_session. Defaults to :exception.

    • :exception - for invalid requests, this plug will raise Plug.CSRFProtection.InvalidCSRFTokenError.
    • :clear_session - for invalid requests, this plug will set an empty session for only this request. Also any changes to the session during this request will be ignored.

Disabling

You may disable this plug by doing Plug.Conn.put_private(:plug_skip_csrf_protection, true). This was made available for disabling Plug.CSRFProtection in tests and not for dynamically skipping Plug.CSRFProtection in production code. If you want specific routes to skip Plug.CSRFProtection, then use a different stack of plugs for that route that does not include Plug.CSRFProtection.

Examples

plug Plug.Session, ...
plug :fetch_session
plug Plug.CSRFProtection

Link to this section Summary

Functions

Callback implementation for Plug.call/2

Deletes the CSRF token from the process dictionary

Gets the CSRF token

Callback implementation for Plug.init/1

Link to this section Functions

Callback implementation for Plug.call/2.

Deletes the CSRF token from the process dictionary.

This will force the token to be deleted once the response is sent.

Gets the CSRF token.

Generates a token and stores it in the process dictionary if one does not exist.

Callback implementation for Plug.init/1.