Bypassing the Same-Origin-Policy For Local Files During Development

downloadable font: download failed …: status=2147500037

Are you ever stumbled accross weired errors with font-files, that could not be loaded, or SVG-graphics, that are not shown during local development on your machine using file:///-URI’s, though everything works as expected, if you push the content to a webserver and access it via HTTP? Furthermore, the browsers behave very differently here. Firefox, for example, just states, that the download of the font failed:


downloadable font: download failed (font-family: "XYZ" style:normal weight:normal stretch:normal src index:0): status=2147500037 source: file:///home/you/path/to/font/xyz.woff

Meanwhile, Chrome just happily uses the same font. Considering the SVG-graphics, that are not shown, Firefox just does not show them, like it would not be able to at all. Chrome logs an error:


Unsafe attempt to load URL file:///home/you/path/to/project/img/sprite.svg#logo from frame with URL file:///home/you/path/to/project/templates/layout.html. Domains, protocols and ports must match

…though, no protocol, domain or port is involved.

The Same-Origin Policy

The reason for this strange behavior is the Same-origin policy. Chrome gives you a hint in this direction with the remark that something does not match. I found the trail, that lead me to this explanation, while googling for the strange error message, that Firefox gives for the fonts, that can not be loaded.

The Same-origin policy forbids, that locally stored files can access any data, that is stored in a parent-directory. They only have access to files, that reside in the same directory or in a directory beneath it.

You can read more about that rule on MDN.

I often violate that rule, when developing templates for dynamically rendered pages with Thymeleaf, or similar techniques. That is, because I like to place the template-files on a subdirectory of the directory, that contains my webapp (src/main/webapp with Maven):


+ src/main/webapp/
  + css/
  + img/
  + fonts/
  + thymeleaf/templates/

I packed a simple example-project for developing static templates with LESS, nodejs and grunt, that shows the problem and the quick solution for Firefox presented later. You can browse it on my juplo.de/gitweb, or clone it with:


git clone http://juplo.de/git/examples/template-development

Cross-Browser Solution

Unfortunately, there is no simple cross-browser solution, if you want to access your files through file:///-URI’s during development. The only real solution is, to access your files through the HTTP-protocol, like in production. If you do not want to do that, the only two cross-browser solutions are, to

  1. turn of the Same-origin policy for local files in all browsers, or
  2. rearrange your files in such a way, that they do not violate the Same-origin policy (as a rule, all resources linked in a HTML-file must reside in the same directory as the file, or beneath it).

The only real cross-browser solution is to circumvent the problem altogether and serve the content with a local webserver, so that you can access it through HTTP, like in production. You can read how to extend the example-project mentioned above to achieve that goal in a follow up article.

Turn Of Security

Turning of the Same-origin policy is not recommended. I would only do that, if you only use your browser, to access the HTML-files under development ‐ which I doubt, that it is the case. Anyway, this is a good quick test to validate, that the Same-origin policy is the source of your problems ‐ if you quickly re-enable it after the validation.

Firefox:
Set security.fileuri.strict_origin_policy to false on the about:config-page.
Chrome:
Restart Chrome with --disable-web-security or --allow-file-access-from-files (for more, see this question on Stackoverflow).

Quick Fix For Firefox

If you develop with Firefox, there is a quick fix, to bypass the Same-origin policy for local files.

As the explanation on MDM stats, a file loaded in a frame shares the same origin as the file, that contains the frameset. This can be used to bypass the policy, if you place a file with a frameset in the topmost directory of your development-folder and load the template under development through that file.

In my case, the frameset-file looks like this:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
<html>
  <head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8">
    <title>Frameset to Bypass Same-Origin-Policy
  </head>
  <frameset>
    <frame src="thymeleaf/templates/layout.html">
  </frameset>
</html>

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