25 March 2015

Tags: clojure buddy security

Part 4 in my blog series about securing clojure web services using buddy. The time has finally come to demonstrate how you may secure a REST based microservice application.


Sample code (tagged for each blog post) can be found on github

Before I discovered buddy my first attempt at prototyping a clojure web app with security tried to combine the use of Friend and Liberator. To complicate matters I tried to make an app that both served user content (html) and provided a REST api. I had a hard time figuring out how to make the two play nicely together. If it hadn’t been for the brilliant article: API Authentication with Liberator and Friend by Sam Ritchie, I wouldn’t have gotten very far.

In this episode I will try to demonstrate how you may use buddy in combination with Liberator to secure a REST-oriented microservice application. We are going to build upon the token based authentication and authorization from the previous episodes and create the acme-catalog service app.

A small contribution to buddy

The primary buddy lib to help you secure ring based web apps is buddy-auth. Unfortunately when I first wanted to use buddy-auth for authentication and authorization, it didn’t provide out of the box support for jws tokens. What to do ? Well I decided to do what any good open citizen should do. I submitted a pull request. My first clojure lib contribution got accepted. Yay !

Relevant code snippets for acme-catalog

Wrap authentication middleware

(ns acme-catalog.core
  (:require [compojure.core :refer [defroutes ANY]]
            [ring.middleware.params :refer [wrap-params]]
            [ring.middleware.keyword-params :refer [wrap-keyword-params]]
            [ring.middleware.json :refer [wrap-json-params]]
            [clojure.java.io :as io]
            [buddy.auth.backends.token :refer [jws-backend]]
            [buddy.auth.middleware :refer [wrap-authentication]]
            [buddy.core.keys :as ks]
            [acme-catalog.resources :as r]))

(defroutes app-routes
  (ANY "/products" [] r/products)
  (ANY "/products/:id" [id] (r/product id)))

(def auth-backend (jws-backend {:secret (ks/public-key (io/resource "auth_pubkey.pem"))  (1)
                                :token-name "Acme-Token"}))

(def app
  (-> app-routes
      (wrap-authentication auth-backend)                                                 (2)
1 Buddy auth backend that supports jws tokens. We provide the public key for the certifacate used by acme-auth to create our tokens. In addition we can optionally provide a custom name for our token
2 Apply middleware that uses backend to read the token, unsign it and populate request map with the token info
What does the wrap-authentication middleware do ?
  1. Retrieves the Authorization header (if one exist) from the request headers

  2. Looks for the token param (default "Token", but in our case "Acme-Token")

  3. If found reads the token and unsigns it using the secret (in our case the public key)

  4. The contents of the unsigned token is added to an :identity key in your request map

Sample request map
    {:user-roles [{:role-id 10, :application-id 10}
                  {:role-id 41, :application-id 40}],
     :username test, :id 1},
 :exp 1427285979},
 ;; etc...

Authorizing liberator resources

Liberator routes your request through a graph of decisions and actions. This graph provides a useful context in case you are not familiar with what decisions kicks in when !

I initially tripped on the difference between HTTP status 401 and 403. Stackoverflow provides a pretty clear explanation.

Helper functions for role access

(def acme-catalog-roles
  {:customer 41 :catalog-admin 40})                                       (1)

(defn any-granted? [ctx roles]                                            (2)
    (set (map :role-id (-> ctx :request :identity :user :user-roles)))
    (set (vals (select-keys acme-catalog-roles roles))))))
1 Hardcoded definition of roles applicable for the acme-catalog app
2 Helper function to check if the user has been granted one or more of the applicable roles

Liberator resource commons

Liberator resources are composable, so to avoid too much repetion across resources we’ve created a small helper function to define behavior for the two key decision points with regards to authentication and authorization checks.

(defn secured-resource [m]
  {:authorized?    #(authenticated? (:request %))                                       (1)
   :allowed?       (fn [ctx]
                     (let [default-auth? (any-granted? ctx (keys acme-catalog-roles))]  (2)
                       (if-let [auth-fn (:allowed? m)]
                         (and default-auth? (auth-fn ctx))                              (3)
1 :authorized? corresponds to 401. Here we check if the user is authenticated. We use a buddy function: authenticated? to do the check. If the user isn’t authentication this function will return false
2 :allowed? corresponds to 403. We provide a default impl here that says that the user must atleast have one of the acme-catalog roles to be authorized to access a secured resource
3 In addition we provide an optional facility to specify a custom function for more fine grained authorization checks. See below for example.

Securing products resources

(defresource product-categories
  (secured-resource {})                                                                   (1)
  :available-media-types ["application/json"]
  :allowed-methods       [:get]
  :handle-ok             (fn [ctx] "List of categories"))

(defresource products
  (secured-resource {:allowed? (by-method {:get true                                      (2)
                                           :post #(any-granted? % [:catalog-admin])})})
  :available-media-types ["application/json"]
  :allowed-methods       [:get :post]
  :handle-ok             (fn [ctx] "List of products coming your way honey"))

(defresource product [id]
  (secured-resource {:allowed? (by-method {:get true
                                           :delete #(any-granted? % [:catalog-admin])
                                           :put #(any-granted? % [:catalog-admin])})})
  :available-media-types ["application/json"]
  :allowed-methods       [:get :put :delete]
  :handle-ok             (fn [ctx]                                                        (3)
                           (if (and (= "99" id)
                                    (not (any-granted? ctx [:catalog-admin])))
                             (ring-response {:status 403
                                             :headers {}
                                             :body "Only admins can access product 99"})
                             "A single product returned")))
1 For the product-categories service anybody with a acme-catalog role may access
2 For products we restrict access by request method. Only catalog admins may add new products, while anyone can list products.
3 Silly example, but demonstrates that you can always bypass the defaults and do custom authorization further down in the liberator decision chain.

Trying it all out - commando style

Get a valid token
acme-auth: lein ring server-headless

# In another terminal
curl -i -X POST -d '{"username": "test", "password":"secret"}' -H "Content-type: application/json" http://localhost:6001/create-auth-token

# Responds with something like:
HTTP/1.1 201 Created
Date: Wed, 25 Mar 2015 11:49:39 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 1057
Server: Jetty(7.6.13.v20130916)

Calling acme-catalog
acme-catalog: lein ring server-headless

#in another terminal
curl -i -H "Authorization: Acme-Token eyJ0eXAiOiJKV1MiLCJhbGciOiJSUzI1NiJ9.eyJ1c2VyIjp7InVzZXItcm9sZXMiOlt7InJvbGUtaWQiOjEwLCJhcHBsaWNhdGlvbi1pZCI6MTB9LHsicm9sZS1pZCI6NDEsImFwcGxpY2F0aW9uLWlkIjo0MH1dLCJ1c2VybmFtZSI6InRlc3QiLCJpZCI6MX0sImV4cCI6MTQyNzI4NTk3OX0.eNTNG8Hu8a4OD9xWSoEZgwGUd15Oytj-GQZY4RgmTEdx9OjkLDRBefU89GNlEEq19Bsd3ciuWzTXKg3B0qvAk4F4-najY_erPGypSlBvRUI0Fa1_wA2PRYxT-zCTiSIxD-oM0oq_3Z61QlN0k-Sf7shel42-x9z7r8RQeNMr-iMk-hOI_v7moQogN08FiZnctcQdE8qKg_DEhwO3l780eBta_vr3tGSd174IRthz59G61P-XqV8wC4HZymbe8TCMc-3uniIvQeoG_rC3oRqNfjkxZlTB_h6mOjs1p3h_cUmrsOhSk0mQe5mrwSzuCiunMcKQ1jsb88daWkvjMrwRUg" http//localhost:6003/products/1

# reponds with something like
HTTP/1.1 200 OK
Date: Wed, 25 Mar 2015 13:47:50 GMT
Vary: Accept
Content-Type: application/json;charset=UTF-8
Content-Length: 25
Server: Jetty(7.6.13.v20130916)

A single product returned

Integration with acme-webstore

Calling acme-catalog from acme-webstore should now be a pretty simple matter. We just need to make sure we pass on the token.

Calling acme-catalog

(ns acme-webstore.catalog
  (:require [clj-http.client :as http]))

(defn get-from-catalog [path token]
  (http/get path {:headers {"Authorization" (str "Acme-Token " token)}}))       (1)

(defn get-products [req]
  (let [auth-token (-> req :session :token-pair :auth-token)                    (2)
        resp (get-from-catalog "http://localhost:6003/products" auth-token)]
    (:body resp)))
1 We make sure we pass the token in the Authorization header with the given token name
2 The auth-token for the logged in user is found under the session key for the request

The rest is just a matter of hooking up the appropriate route and view. I’ll leave that part up to you !


Most of the hard work was already done in the previous episodes. Providing authentication and authorization for our REST services was pretty simple. We also demonstrated that integrating with Liberator was mostly a matter of hooking into the appropriate decision points for our resource definitions. We didn’t utilize all that much of buddy-auth here, but your app might find use for some of its more advanced features.

I think this episode demonstrates some of the benefits of using a library like buddy. It’s not very opnionated which leaves you with a lot of decisions to make. But it does have the building blocks you need and it provides you with great flexibility when it comes to integrating with other libraries.

At the moment I’m not sure if there is going to be any further episodes in the near future. But the again it might. Feel free to leave suggestions in the commenting section though.

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