Security Introduction

• Session Security
• Form Method Spoofing
• File Uploads

Coast provides a handful of tools to keep your websites secure from common web attacks.

In this guide, we learn about the best practices to keep your Coast applications secure.

Session Security

Sessions can leak important information if not handled with care.

Coast encrypts and signs all cookies using the :session-key defined in the env.edn file.

Keep your production :session-key secret – don't share it with anyone, and never push it to version control systems like Github.

NOTE: Do not re-use the env.edn file's :session-key in production. Come up with a new 16-byte key

Session Config

Session configuration can be specified as a map passed to app in the src/server.clj file.

When updating your session configuration, considering the following suggestions:

(coast/app {:session {:cookie-attrs {:http-only true, :same-site :strict}}})

These are Coast's default settings in development and production

• The :http-only value should be set to true, as setting it to false will make your cookies accessible via JavaScript using document.cookie.
• The :same-site value should be set to :strict, ensuring your session cookie is not visible/accessible via different domains.

Form Method Spoofing

As HTML forms are only capable of making GET and POST requests, you cannot use HTTP verbs like PUT or DELETE to perform resourceful operations via a form's method attribute.

To work around this, Coast implements method spoofing, enabling you to send your intended HTTP method via the request's hidden _method input (in a form):

[:put "/customers/:customer-id" :customer/update]

[:form {:method "post" :action "/customers/123"}
  [:input {:type "hidden" :name "_method" :value "put"}]]

; or with form-for
(form-for :customer/update {:customer-id 123})

In the example above, adding a hidden input with the name _method and value put to the form body converts the request HTTP method from POST to PUT.

Here are a couple of things you should know about method spoofing:

• Coast only spoofs methods where the source HTTP method is POST, meaning GET requests passing an intended HTTP _method are not spoofed.

File Uploads

Attackers often try to upload malicious files to servers to later execute and gain access to servers to perform some kind of destructive activity.

Besides uploading malicious files, attackers may also try to upload huge files so you server stays busy uploading and starts throwing TIMEOUT errors for subsequent requests.

To combat this scenario, Coast lets you define the maximum upload size processable by your server. This means any file larger than the specified :max-body is denied, keeping your server in a healthy state.

Set your :max-body value as bytes inside the app map in server.clj:

; src/server.clj

(def app (coast/app {...}))

(coast/server app {:max-body 8388608}) ; the default of 8 MB

Here are a few tips to consider when handling file uploads:

• Rename files before uploading/storing. Typically to a uuid + file extension.
• Don't store uploaded files inside the public directory, since public files can be accessed directly.
• Don't share the actual location of uploaded files with your users. Instead, consider saving a reference to uploaded file paths in your database (each file having a unique id), and set up coast's built in :storage middleware to serve those uploaded files via that identifier, like so:

(def app (coast/app {:storage (coast/env :file-path)}))