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Open Web Apps: Refining The Manifest
2010-11-22 00:00:00 -0800

In the month since we announced “Open Web Apps”, there’s been a lot of discussion around the particulars of the Mozilla proposal.

I specifically wanted to take a minute to jot down some of the proposed changes to the application manifest format from our initial design. The changes detailed here range from the drastic to the mundane, and have been contributed by my co-workers at mozilla and several community members.

NOTE: ALL of these changes are currently just ideas in want of review, and my listing them here in no way implies that they’ll be adopted.

#1 defaultLocales to default_locales

Underbars are used more predominantly in the manifest to separate multiple word property names, we should make that convention consistent.

#2 Removal of app_urls

The motivation of “application urls” was to define the scope of an application for two different purposes:

  1. navigation support: When clicking around on links inside an application, it is useful if the user agent can know whether a url is within an application. This allows the UA to implement some potentially useful behaviors that wouldn’t otherwise be possible (like keeping installed apps inside firefox ‘app tabs’ or google chrome’s ‘pinned tabs’).
  2. capability scoping: For applications which request different capabilities (or “permissions”), the app_urls array defined the scope of those capabilities. That is, the act of installing an application bestows those permissions on pages residing at URLs which match members of app_urls.

While the inclusion of app_urls may be useful for making navigation support possible for existing web applications, the risks it involves are too great. Specifically, if an application is able to request that capabilities are bestowed on arbitrary domains, several new types of attacks are possible.

One simple and concrete example of an attack could involve a hypothetical ‘navigation_consolidation’ capability. Imagine that an application existed and was designed to be a “singleton”: That application could request that if the user navigates to urls inside the app, rather than opening a new instance of the application, the existing instance of the app should be notified of the user’s navigation and the tab or window containing it focused.

This navigation_consolidation feature when combined with a broad specification of app_urls could offer a malicious application the ability to intercept user’s navigation to sites with whom they share personal or financial information (open the doors for a new and dastardly type of phishing attack).

This is one specific example of a class of problems that arises when an application author is able to change the way the browser behaves when rendering content from a site that that author does not control.

Part of a solution to this class of attacks is to simply remove the app_urls property from a manifest, in effect making it impossible for an application to span multiple domains. Once app_urls is removed, however, how is application scope determined? The next proposal attempts to answer that question.

#3 base_url defines “application scope”

Having removed app_urls in #2 it becomes unclear how, given a url, one determines whether that url belongs to an application. A simple solution is to say that any resource which has the base_url parameter of an application as a prefix is considered within the scope of that application.

A problem of overlapping application scope still exists: specifically if one app contains a base_url which actually contains the base_url of other applications, there is scope ambiguity. This problem is mitigated by the removal of app_urls (which could make scope ambiguity more prevalent and harder to reasonably address), and a reasonable solution may for the UA to simply refuse to install applications which would introduce ambiguity. This solution means that it’s possible for a rogue application to block the installation of others, but because the conflicting applications must always be from the same principle, it should be simple to resolve any such issues.

#4 Canonical Manifest URLs

Aaron Boodman of google suggested in an email that manifests be “served from a live URL, rather than embedded in … page(s)”. There are many reasons why this suggestion is interesting:

  1. (via Boodman) Every application has a “clear and unique ID”
  2. (via Boodman) Convenience: a developer may simply post their manifest URL to stores.
  3. Ownership: When combined with the requirement that urls inside a manifest must match the url from which the manifest is served, it builds a simple mechanism of proof of ownership into the system.

#5 Addition of a manifest_name property

Given the presence of base_url if a manifest_name were added to the manifest, it would mean that each manifest suddenly has a canonical url, which is manifest.base_url + manifest.manifest_name. This canonical url can be used to verify an application (see #6) and can be used as a unique identifier of the application.

manifest_name might be optional, with manifest.json being its default value.

One security feature of manifest_name, could be that the name must be a top level entity within the path described by base_url. This feature could address a class of attacks which arise when sites host user generated content. While it may be possible to publish a manifest on an unaware vulnerable site, it would not be possible to specify a base url within that injected manifest that included more than the directory from which the UGT was served (yeah, this only mitigates this type of attack, and we still may at some point consider stronger protections, such as new HTTP headers).

#6 Application verification at installation time

Combining #2 and #3 above, it becomes possible to define a series of steps that the application repository can perform to “verify an application”. If an application is verified, that means that the manifest resides on the site that hosts the application. This property is a speed-bump to many attacks, in that it’s no longer possible to casually grant permissions to a site you do not control.

The process of verification is simple:

  1. if installation was triggered with a manifest, extract the full canonical path to the manifest: base_url + manifest_name
  2. using the url passed to install() or the result of step #1 fetch the contents at that location
  3. if what’s fetched is not a valid manifest then the application cannot be verified
  4. extract base_url and manifest_name from the manifest fetched, if that matches the url from which the manifest was fetched, then the app is considered valid

Given this process to check that a manifest author has control over domain in which her web application resides, we can choose to either prevent the installation of unverified applications, or at the very least prevent them from requesting any capabilities outside of what’s available to a standard web application.

#7 installed_by

In the current proposal there is no way for a host of an application to explicitly delegate authority to a store to provision that application. In other words, if I wrote “Whap The Monkey” and want to allow “http://redonkulousgames.com” to legitimately sell my game, there’s no way that I can inform your UA that you are allowed to install WTM from my site and the redonkulousgames.com site, but from noone else.

Given the suggestions in this post, it could now be fairly trivial to specify in the manifest who may distributed an application. installed_by could simply be an array of url fragments, with the default being “disallow syndication”:

"installed_by": [ ]

The developer may explicitly allow promiscuous syndication of their app:

"installed_by": [ "*" ]

And the whap the monkey case of above could look something like:

"installed_by": [ 

This simple addition further empowers application developers (or app publishing platforms) with more control to protect apps against casual (even accidental) plagiarism, and unwanted syndication. Further, the UA can enforce this mechanism to eliminate several new flavors of phishing.

#8 anonymous apps

While the above proposals tighten up the definition of an application and attempt to eliminate several areas of ambiguity, they do so at the sacrifice of an interesting feature: How might I publish a simple manifest that describes an existing website, so that I may launch that site from within my dashboard?

One solution is to create explicit support for “anonymous apps” (which are simply applications that the UA cannot verify with the steps in #6), these are applications which may not request “capabilities”, and in turn need not be hosted anywhere (all icons can be embedded in the manifest itself in the form of data urls, and hence the base_url property becomes unnecessary).

This approach aims to allow for the distributed development of “bookmark applications” apps in a safe way (allowing end users to have a convenient way to launch the applications that are important to the even if the site authors haven’t yet built any explicit support in for open web apps).


The focus of most of these features is to tighten up web apps and to reduce ambiguity and attack surface in preparation for more interesting and powerful capabilities. So while it may seem that some of these restrictions limit the power of web applications, this author believes that a reduced application scope will make it easier and safer once we start adding application capabilities.

What do you think?