Jul 09, 2015

Securing your clojurescript app

Use buddy's json tokens to authenticate your single page application

author picture
Francesco Sardo
Senior Software Engineer

The Web After Tomorrow will be populated by apps: standalone, in-browser, Javascript applications. Mobile devices set a high bar for user experience that we’re now demanding from the web: we want interfaces that provide instant feedback, are always up to date and gracefully degrade when offline (just to name a few).

Just like a mobile app is concerned about visualization, user interaction, caching and syncing so will be the next generation of JS apps. “Web” is just another platform to target along with iOS and Android, with its own apis, quirks and strengths, but with no special treatment.

Clojurescript has great potential to shine in this scenario. Not only is the language well suited to manage the inherent complexity of a rich client software, but the tools commonly found in a Clojure stack (Datomic, core.async, transit, Om) are already designed for a modern approach to applications development. Single page Clojurescript apps matter .

Say form submission once again

In such a world you can serve your application from any generic static web hosting such as Amazon S3 or specialised ones like divshot. Your app would access and consume any number of services: twitter, slack, github, dropbox, etc. Your server and database, if any, would not be a special case anymore.

So if you still need to deploy a server and a database to store users and stuff, why would you set up something so ad-hoc like Http Session to manage authentication? Do like any other service that you integrate would do and use Json Tokens instead.

There are plenty of resources that already discuss in detail what these tokens are and why you should use them, so I’m just going to summarise it for people too lazy to follow links.

In a nutshell this is how token authentication works:

  • Client sends username and password to the server.

  • Server checks credentials (e.g. db lookup). If valid, the server encrypts some information into a token string that is returned to client as a body payload.

  • Client saves the token somewhere and sends it as Authorization header on every request to Server.

  • Server decrypts the authorization header. If decryption is successful it means client is authenticated and can proceed.

And it’s great for these reasons (among others):

  • It’s not necessary to go back to the database on every request and validate the username and password. The token string grants you access until it expires, no other checks are needed.

  • You can use exactly the same mechanism for your mobile clients.

  • It’s stateless (no need to keep sessions anywhere, the server can scale at will).

  • The authentication machinery can be moved to a separate microservice. As long as the other services can decrypt the authorization map they don’t even need to know there’s a user database somewhere.

  • Tests are easier (no need to setup cookie stores) and it works great with tools like swagger.

Show me the code already

Most people know and use friend as a security library for Clojure, but that’s not the only one. Buddy is an excellent alternative, provided with extensive documentation and modular structure, which means it’s not tightly coupled to Ring. That’s good, because the following example uses pedestal (it oubviously works just as well with Ring).

(ns app.users
  (:require [io.pedestal.impl.interceptor :refer [terminate]]
            [io.pedestal.interceptor.helpers :refer [defbefore defhandler]]
            [datomic.api :as d]
            [clj-time.core :refer [hours from-now]
            [buddy.auth.protocols :as proto]
            [buddy.auth.backends.token :refer [jwe-backend]]))

(defonce secret "my-secret")

(def encryption {:alg :a256kw :enc :a128gcm})

(defn- user-info [db user]
  (d/pull db [:db/id :user/role :user/company] user))

(defhandler login
  [{:keys [body-params db] :as request}]
  (let [{:keys [username password]} body-params
        [user] (d/q '{:find [[?e]]
                      :in [$ ?username ?password]
                      :where [[?e :user/email ?username]
                              [?e :user/password ?enc]
                              [(buddy.hashers/check ?password ?enc)]]}
                    db username password)
        valid? (some? user)]
    (if-not valid?
      {:status 401 :body {:message "Wrong credentials"}}
      (let [info (user-info db user)
            claims {:user info
                    :exp (-> 3 hours from-now)}
            token (jwe/encrypt claims secret encryption)]
        {:status 200 :body {:token token}}))))

The important bit is the line (jwe/encrypt claims secret encryption). After we’ve successfully authenticated the user we create a claims map. The most relevant key is the expiration (set at three hours in the example above) otherwise the user will be forever authenticated. This claims map can be merged with any other data that you might want to get back when you decrypt the json token sent by the client. In the example we’re extracting the user info from a Datomic reference.

When the client sends back the token we can check the authentication and assoc the identity to the request without an extra round trip.

(defbefore check-auth
 [{:keys [request] :as context}]
  (let [req (try (some->> (proto/parse auth-backend request)
                          (proto/authenticate auth-backend request))
                 (catch Exception _))]
    (if (:identity req)
      (assoc context :request req)
      (-> context
          (assoc :response {:status 401 :body {:message "Unauthorized"}})))))

(_Thumbs up for pedestal interceptors for making it easy to early terminate a request if the authentication fails, instead of the usual try/catch/error-fn dance required by ring middlewares. _)

You can place this interceptor at the root of an authenticated route and any path under it will see the decrypted token under the :identity key.

Now that you know if a user is authenticated, you probably want an interceptor that checks if the user is allowed to access that resource.

(defn- user-allowed? [user path-params]
 (let [{company-path :company} path-params
       {:keys [:user/role :user/company]} user]
   (and (= role :admin) (= company company-path))))

(defbefore check-permissions
 [{:keys [request] :as context}]
  (let [{:keys [db identity path-params]} request
        user (:user identity)
        allowed? (user-allowed? user path-params)]
    (if allowed?
      (-> context
          (assoc :response {:status 403 :body {:message "Not allowed"}})))))

The function user-allowed? obviously depends on your business domain, but the concept is the same: we’re checking if the user represented by {:db/id X :user/role Y :user/company Z} can access a specific resource. Placing check-permission at the root of some paths will short circuit the request if the user is not allowed to call that handler.

Here’s how everything plays together in with pedestal’s route definition.

(defroutes routes
     ["/login" {:post login}]
     ["/user" ^:interceptors [check-auth] {:get profile}
      ["/:company/transactions" ^:interceptors [check-permissions] {:get transactions}]]]]])


If you’re doing user authentication in your Clojurescript app you might want to switch to Json Tokens instead of sticking with Http Session to make the server simpler to use, scalable and mobile ready. Json Tokens are approved by Master Pai Mei himself.

Discuss below or join the discussion on Hacker News.

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