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Sheriff is a simple minimal-dependency way to manage policy based authorization for Plug based applications, including Phoenix.

If you're looking for authentication check out Guardian or Ueberauth for third-party oauth.

Installation

The latest on hex.pm:

def deps do
  [{:sheriff, "~> 1.0"}]
end

If you prefer living on the edge use master:

def deps do
  [{:sheriff, github: "beam-community/sheriff"}]
end

Current User

By default Sheriff will look for the current user in the :current_user key within Plug.Conn.private. Since this may not be compatible with all applications, we've provided a way to reconfigure the key:

config :sheriff,
  resource_key: :your_desired_resource_key

Resource Loading

Resource loaders are responsible for retrieving the requested resources. A global loader can be specified in your application configuration or individual loaders can be supplied on a per plug basis.

Sheriff ships with a convenient Sheriff.ResourceLoader behaviour:

defmodule Example.UserLoader do
  @behaviour Sheriff.ResourceLoader

  def fetch_resource(:show, %{"id" => id}), do: Repo.get(User, id)
  def fetch_resource(:index, _params), do: Repo.all(User)
end

Laws

In Sheriff authorization is handled with laws; which are modules that implement the Sheriff.Law behaviour:

defmodule Example.UserLaw do
  @behaviour Sheriff.Law

  alias Example.User

  # Admins can see all the things!
  def legal?(%User{role: "admin"}, _request, _resource), do: true

  # Users can access themselves
  def legal?(%User{id: id}, _request, %User{id: id}), do: true

  # Team admin can view team members
  def legal?(%User{role: "team_admin", team_id: id}, :show, resources) do
    Enum.all?(resources, &(&1.team_id == team_id))
  end

  # No match, no access
  def legal?(_, _, _), do: false
end

Plugs

There are two plugs that serve as the workhorses of Sheriff, these need to occur after Plug.Parser:

  • Sheriff.LoadResource - Uses the configured ResourceLoader to fetch the target resource
  • Sheriff.LawEnforcer - Apply a given Law against the current user, target resource, and request.

When defining our authorization pipeline we could use something like this:

plug Sheriff.LoadResource, loader: Example.UserLoader
plug Sheriff.LawEnforcer, law: Example.UserLaw

Note: The Sheriff.LoadResource and the Sheriff.ResourceLoader behaviour are for simple use-cases. For more involved resource fetching you may find it's necessary to implement your own plug, make sure the loaded resources are assigned to the connection under the :sheriff_resource key.

Error Handling

Sheriff has just three error scenarios we need to address:

  • The requested resource is missing
  • The current user is not authenticated
  • The current user is not authorized to perform the requested action

To handle these cases we'll want to provide an error handler for Sheriff. Our handler can be a module that implements resource_missing/1, unauthenticated/1, and unauthorized/1; the Sheriff.Handler behaviour is optional.

Sheriff makes no assumptions so we need to tell it which module to use as a handler:

config :sheriff,
  handler: Example.ErrorHandler

That's it!

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Build simple and robust authorization systems with Elixir and Plug

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