How to safely embed JSON with in HTML document?

Question

In a Rails 3.1 app, how can I safely embed some JSON data into an HTML document?

Suppose I have this in a controller action:

@tags = [
    {name:"tag1", color:"green"}, 
    {name:"</script><b>I can do something bad here</b>", color:"red"}
]

And this in a corresponding view:

<script type="text/javascript" charset="utf-8">
    //<![CDATA[
    var tags_list = <%= @tags.to_json %>;
    // ]]>
</script>

Then I get this in resulting HTML:

var tags_list = [
    {"name":"tag1","color":"green"},
    {"name":"</script><b>I can do something bad here</b>","color":"red"}
];

which triggers a SyntaxError: Unexpected token & in Chrome

If I remove the Rails' default HTML escaping with <%=raw tags.to_json %>, then it returns this:

var tags_list = [
    {"name":"tag1","color":"green"},
    {"name":"</script><b>I can do something bad here</b>","color":"red"}
];

which, of course, breaks the HTML document with </script>.

Can I somehow tell to_json() method to return something more like this:

var tags_list = [
    {"name":"tag1","color":"green"},
    {"name":"</script><b>I can do something bad here</b>","color":"red"}
];

I asked this question on rubyonrails-talk mailing list, and I understand now that some people think that's a very bad idea to begin with, but in my case it works very nicely, as long as there are no HTML special chars in the data. So I just want to make the string returned by to_json HTML safe and still have JavaScript parse it properly.

UPDATE: Based on @coreyward comment, I did make it a JS string literal, and that seems to be working great now. Its not quite as elegant of a solution as I was hoping for, but its not too bad either. Here is the code that is working for me:

<% tags = [{name:"tag1", color:"green"}, {name:"</script><b>I can \n\ndo something bad here</b>", color:"red"}] %>

<script type="text/javascript" charset="utf-8">
    //<![CDATA[
    var tags_list = $.parseJSON('<%=j tags.to_json.html_safe %>');
    // ]]>
</script>

which results in:

<script type="text/javascript" charset="utf-8">
    //<![CDATA[
    var tags_list = $.parseJSON('[{\"name\":\"tag1\",\"color\":\"green\"},{\"name\":\"<\/script><b>I can \\n\\ndo something bad here<\/b>\",\"color\":\"red\"}]');
    // ]]>
</script>

Answer 1

Your code using just @tags.to_json works in rails3, if you enable it with:

   ActiveSupport.escape_html_entities_in_json = true

Otherwise, your other option is this:

   var tags_list = <%= raw @tags.to_json.gsub("</", "<\\/") %>;

This saves the client having to parse the whole thing through $

Answer 2

The proper way in 2019 is to wrap obj.to_json with json_escape function. json_escape is directly intended for escaping specific HTML symbols inside JSON strings. Example below from the documentation:

json = JSON.generate({ name: "</script><script>alert('PWNED!!!')</script>"})
# => "{\"name\":\"</script><script>alert('PWNED!!!')</script>\"}"

json_escape(json)
# => "{\"name\":\"\\u003C/script\\u003E\\u003Cscript\\u003Ealert('PWNED!!!')\\u003C/script\\u003E\"}"

JSON.parse(json) == JSON.parse(json_escape(json))
# => true

It seems this page appears on top of Google Search results, that's why I decided to provide a comment with an update :)

Answer 3

btw, this works but is not a good solution in my opinion:

<script type="text/javascript" charset="utf-8">
  //<![CDATA[
  var tags_list = <%=raw @tags.to_json.gsub('/', '\/') %>;
  // ]]>
</script>

Answer 4

I think that if you try this it will work:

var tags_list = "<%== @tags.to_json.gsub('/', '\/') %>";

(Notice the double == and the " ")

Answer 5

For instance with this in app/layouts/application.html.slim:

    javascript:
      window.translations = #{raw t("js").to_json};

And this in the translations:

  js:
    name:
      must_be_present: Must be present<script>alert(1)</script>

The result will be escaped:

<script>window.translations = {"name":{"must_be_present":"Must be present\u003cscript\u003ealert(1)\u003c/script\u003e"}};</script>