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From Ian Clelland <iclell...@chromium.org>
Subject How to handle CSP for XHR in Cordova 4.0
Date Fri, 12 Dec 2014 14:13:48 GMT
I'm just building the new optional whitelist plugins for Cordova Android
and iOS 4.x, and I'm thinking about how to encourage developers to use CSP
for network requests, as opposed to a
Cordova-implemented-whitelist-which-probably-leaks-like-a-sieve.

(Note: This is really just about things like XHR requests, <img> and
<script> tags, etc, which are historically the only things that we've
reliably been able to filter out. Other classes of network requests just
bypass all of our code anyway, which sucks, and frame navigation and
external application launches are already well handled by the framework).

The policy I've implemented on the unplug-whitelist branches, which at
first thought at least *sounds* sane, is that network requests are blocked
by default. (At least all of the ones that we can intercept). That way, a
plugin, such as the legacy-whitelist plugin, can open up access to specific
resources, and the fallback is safety.

To use CSP, though, we have to open up access to the outside, and we don't
necessarily know what the developer wants to open (the whole point is that
they specify in the HTML, not in a config file.) The easiest way is to open
up access to *all* resources to the webview, and then restrict it through
the CSP header/meta-tag, which does a better job of blocking those requests
than we do in any case.

I think that we want to encourage developers to use CSP, for a lot of
reasons, but I'm going to have to do one of these things, and I'm not
entirely sure which is the right one:

1. Open access to all network resources by default in Cordova 4.x.
  * This doesn't apply to navigations, or launching other apps. They're
still blocked by default.
  * Any plugin implementing shouldAllowRequest would still be able to turn
this into a disallow-by-default whitelist
  * We can't block everything anyway (see websockets, audio/video streams /
probably more), so it removes the illusion that we can.

2. Make another whitelist-y plugin, something like "org.apache.cordova.csp"
that exists only to open up access to network resources. Direct all users
who want to use CSP to install that plugin first
  * It's a 4th network plugin (after legacy-whitelist, navigation-whitelist
and intent-whitelist)
  * Adding it doesn't actually add any CSP protection, so it probably needs
a better name
  * It's an extra step that may confuse people and limit adoption

3. Do something crazy. Maybe a CSP plugin that automatically creates CSP
tags *and* updates the XHR whitelist, both from config directives.
  * Lots more work
  * We probably don't know enough about real requirements to get this right
  * If CSP is doing its job, then the XHR whitelist isn't needed anyway; it
would just be another layer that isn't doing anything different.

I'm leaning towards #1, but its a it's a decision that we really should
think about and decide on-list before moving forward.

Ian

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