Helpers: Markup Escape


Views escape automatically the output of their methods. There are complex situations that views can’t cover properly and that require an extra attention from us.

Hanami makes available a set of escape helpers, to increase the security of our web applications. They are public methods that are available both in views and templates.

Escape HTML Contents

It’s a method called #escape_html (aliased as #h), that escapes the input.

<p><%= h "<script>alert('xss')</script>" %></p>

Returns

<p>&lt;script&gt;alert(&apos;xss&apos;)&lt;&#x2F;script&gt;</p>

Escape HTML Attributes

HTML attributes are more complex to escape, because they involve attribute delimitation chars (eg. single or double quotes).

We have an extra helper for this specific task: #escape_html_attribute (aliased as #ha) This should be used only when the value of an attribute comes from a user input.

<img src="/path/to/avatar.png" title="<%= ha(user.name) %>'s Avatar">

Whitelisted URLs

Imagine we have a feature in our application that allows users to link from their profile, a website. In the edit profile form we have a text field that accepts a URL.

In the profile page we have a link like this:

<%= link_to "Website", user.website_url %>

A malicious user can edit their profile, by entering javascript code as the website URL. When somebody else clicks on that link, they can receive an XSS attack.

Example:

<a href="javascript:alert('hack!');">Website</a>

The solution to this problem is to wrap the output with the #escape_url (aliased as #hu) helper.

It whitelists URLs that use http, https, and mailto schemes, everything else is scraped.

<%= link_to "Website", hu(user.website_url) %>

In case we need a different set of schemes, we can specify them as second argument.

<%= link_to "Website", hu(user.website_url, ['https']) %>

In the code above, we’re restricting to URLs that only use HTTPS.

Raw Contents

There are cases when we want to print the raw contents. Please be careful with this, because unescaped contents can open a breach for XSS attacks.

The helper is called #raw.

Specific Local

If we want to escape a specific local:

# apps/web/controllers/snippets/show.rb

module Web
  module Controllers
    module Snippets
      class Show
        include Web::Action

        expose :snippet

        def call(params)
          @snippet = "<h1>Hello World</h1>"
        end
      end
    end
  end
end
# apps/web/views/snippets/show.rb

module Web
  module Views
    module Snippets
      class Show
        include Web::View

        def snippet
          raw locals[:snippet]
        end
      end
    end
  end
end
# apps/web/views/snippets/show.html.erb

<%= snippet %>

Entire Output

We can also return unescaped output for the entire view:

# apps/web/views/books/json_show.rb
require 'json'

module Web
  module Views
    module Books
      class JsonShow < Show
        format :json

        def render
          raw JSON.generate(book.to_h)
        end
      end
    end
  end
end