ring-middleware-csp

Ring middleware for Content Security Policy

Installation

To install, add the following to your project :dependencies:

Usage

(require '[ring-middleware-csp.core :refer [wrap-csp]]
         '[ring.util.response :refer [response]])

(defn handler [request]
  (response {:foo "bar"}))

(def policy {:default-src :none
             :script-src [:self :nonce]
             :style-src ["https://example.com" :unsafe-inline]
             :block-all-mixed-content true
             :report-uri "/csp-report"})

(def app
  (-> handler
      (wrap-csp {:policy policy})
      (other-middleware)))

Then, Content-Security-Policy header is added to http response.

Handlers created by wrap-csp support 3-arity versions for asynchronous use.

Use nonce

You can use nonce by setting use-nonce? option to true. wrap-csp middleware inject :csp-nonce to request map. You can use nonce like following.

(defn handler [{:keys [csp-nonce] :as req}]
  {:status 200
   :headers {}
   :body (str "<script nonce=\"" csp-nonce "\">alert('foo');</script>")})

Get header value from policy map

You can use compose function.

(ring-middleware-csp.core/compose {:default-src :none
                                   :style-src ["https://example.com" :unsafe-inline]
                                   :block-all-mixed-content true
                                   :report-uri "/csp-report"})
=> "default-src 'none';style-src https://example.com 'unsafe-inline';block-all-mixed-content;report-uri /csp-report"

; with nonce
(ring-middleware-csp.core/compose {:default-src :none
                                   :style-src [:nonce :unsafe-inline]}
                                  "abcdefg")

=> "default-src 'none';style-src 'nonce-abcdefg' 'unsafe-inline'"

Options

:policy

Specify Content-Security-Policy value. The key of map is the directive name, the value of map is the directive value. Values are keyword, string or collection of them.

e.g.

{:policy {:default-src :none
          :script-src [:self :nonce]
          :style-src ["https://example.com" :unsafe-inline]
          :report-uri "/csp-report"}}

:report-only?

If :report-only? is set to true, use "Content-Security-Policy-Report-Only" as header name.

:policy-generator

By setting a function in :policy-generator, you can set a dynamic policy according to the request. The argument of the function is ring request map, the return value of it is policy map (same style as :policy). If the function returns nil, use default policy.

:report-handler and :report-uri

By using :report-handler, you can handle report request. :report-uri is the path to use report-handler. :report-handler is ring-style report handler (you must return valid response map). If use :report-handler or :report-uri, must set both :report-handler and :report-uri.

WARN: :report-uri option and :report-uri directive in :policy is independent config. Even if you set :report-uri option, the report-uri directive is NOT added automatically.

e.g.

{:policy {:default-src :self
          :report-uri "/csp-report"}
 :report-uri "/csp-report"
 :report-handler (fn [req]
                   (response {:foo "bar"}))}

:use-nonce?

The default value is false. If you set to true, enable to generate nonce.

:nonce-generator

By using :nonce-generator, you can use custom nonce generator. Default generator use SecureRandom (using "NativePRNGNonBlocking" algorithm or, on MS-Windows, the default implementation) and java.util.Base64. It generates base64 string from 256bit random data.

e.g.

{:policy {:default-src :self
          :script-src [:self :nonce]}
 :nonce-generator (fn []
                    "STATIC-NONCE")} ; DON'T use static nonce for security reason

Testing

clojure -M:dev:test

Formatting

Use cljstyle.

cljstyle fix

License

Copyright 2020 Toyokumo,Inc.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.