Firefox or Chrome plugin to block and filter all outgoing connections

Question

In Firefox or Chrome I'd like to prevent a private web page from making outgoing connections, i.e. if the URL starts with http://myprivatewebpage/ or https://myprivatewebpage/ in a browser tab, then that browser tab must be restricted so that it is allowed to load images, CSS, fonts, JavaScript, XmlHttpRequest, Java applets, flash animations and all other resources only from http://myprivatewebpage/ or https://myprivatewebpage/, i.e. an <img src="http://www.google.com/images/logos/ps_logo.png"> (or the corresponding <script>new Image(...) must not be able to load that image, because it's not on myprivatewebpage. I need a 100% and foolproof solution: not even a single resource outside myprivatewebpage can be accessible, not even at low probability. There must be no resource loading restrictions on Web pages other than myprivatewebpage, e.g. http://otherwebpage/ must be able to load images from google.com.

Please note that I assume that the users of myprivatewebpage are willing to cooperate to keep the web page private unless it's too much work for them. For example, they would be happy to install a Chrome or Firefox extension once, and they wouldn't be offended if they see an error message stating that access is denied to myprivatewebpage until they install the extension in a supported browser.

The reason why I need this restriction is to keep myprivatewebpage really private, without exposing any information about its use to webmasters of other web pages. If http://www.google.com/images/logos/ps_logo.png was allowed, then the use of myprivatewebpage would be logged in the access.log of Google's ps_logo.png, so Google's webmasters would have some information how myprivatewebpage is used, and I don't want that. (In this question I'm not interested in whether the restriction is reasonable, but I'm only interested in the technical solutions and its strengths and weaknesses.)

My ideas how to implement the restriction:

• Don't impose any restrictions, just rely on the same origin policy. (This doesn't provide the necessary protection, the same origin policy lets all images pass through.)

• Change the web application on the server so it generates HTML, JavaScript, Java applets, flash animations etc. which never attempt to load anything outside myprivatewebpage. (This is almost impossibly hard to foolproof everywhere on a complicated web application, especially with user-generated content.)

• Over-sanitize the web page using a HTML output filter on the server, i.e. remove all <script>, <embed> and <object> tags, restrict the target of <img src=, <link rel=, <form action= etc. and also restrict the links in the CSS files. (This can prevent all unwanted resources if I can remember all HTML tags properly, e.g. I mustn't forget about <video>. But this is too restrictive: it removes all dyntamic web page functionality like JavaScript, Java applets and flash animations; without these most web applications are useless.)

• Sanitize the web page, i.e. add an HTML output filter into the webserver which removes all offending URLs from the generated HTML. (This is not foolproof, because there can be a tricky JavaScript which generates a disallowed URL. It also doesn't protect against URLs loaded by Java applets and flash animations.)

• Install a HTTP proxy which blocks requests based on the URL and the HTTP Referer, and force all browser traffic (including myprivatewebpage, otherwebpage, google.com) through that HTTP proxy. (This would slow down traffic to other than myprivatewebpage, and maybe it doesn't protect properly if XmlHttpRequest()s, Java applets or flash animations can forge the HTTP Referer.)

• Find or write a Firefox or Chrome extension which intercepts all outgoing connections, and blocks them based on the URL of the tab and the target URL of the connection. I've found https://developer.mozilla.org/en/Setting_HTTP_request_headers and thinkahead.js in https://addons.mozilla.org/en-US/firefox/addon/thinkahead/ and http://thinkahead.mozdev.org/ . Am I correct that it's possible to write a Firefox extension using that? Is there such a Firefox extension already?

Some links I've found for the Chrome extension:

• http://www.chromium.org/developers/design-documents/extensions/notifications-of-web-request-and-navigation
• https://groups.google.com/a/chromium.org/group/chromium-extensions/browse_thread/thread/90645ce11e1b3d86?pli=1
• http://code.google.com/chrome/extensions/trunk/experimental.webRequest.html

As far as I can see, only the Firefox or Chrome extension is feasible from the list above. Do you have any other suggestions? Do you have some pointers how to write or where to find such an extension?

Answer 1

I've found https://developer.mozilla.org/en/Setting_HTTP_request_headers and thinkahead.js in https://addons.mozilla.org/en-US/firefox/addon/thinkahead/ and http://thinkahead.mozdev.org/ . Am I correct that it's possible to write a Firefox extension using that? Is there such a Firefox extension already?

I am the author of the latter extension, though I have yet to update it to support newer versions of Firefox. My initial guess is that, yes, it will do what you want:

1. User visits your web page without plugin. Web page contains ThinkAhead block that would send a simple version header to the server, but this is ignored as plugin is not installed.
2. Since the server does not see that header, it redirects the client to a page to install the plugin.
3. User installs plugin.
4. User visits web page with plugin. Page sends version header to server, so server allows access.
5. The ThinkAhead block matches all pages that are not myprivatewebpage, and does something like set the HTTP status to 403 Forbidden. Thus:
6. When the user visits any webpage that is in myprivatewebpage, there is normal behaviour.
7. When the user visits any webpage outside of myprivatewebpage, access is denied.

If you want to catch bad requests earlier, instead of modifying incoming headers, you could modify outgoing headers, perhaps screwing up "If-Match" or "Accept" so that the request is never honoured.

This solution is extremely lightweight, but might not be strong enough for your concerns. This depends on what you want to protect: given the above, the client would not be able to see blocked content, but external "blocked" hosts might still notice that a request has been sent, and might be able to gather information from the request URL.