W3C home > Mailing lists > Public > public-webappsec@w3.org > August 2012

[CSP] Extensions and user script? (Some feedback)

From: Odin Hørthe Omdal <odinho@opera.com>
Date: Wed, 29 Aug 2012 13:34:32 +0200
To: "public-webappsec@w3.org" <public-webappsec@w3.org>
Message-ID: <op.wjsvruf749xobu@odinho-fido.oslo.osa>
Hello all :-)

I've gotten some internal web site author feedback trying to implement CSP
on a web email service that I'd like to share and discuss.

There's of course a few minor things that will get better with time. Like
browsers using prefixes and different implementations of different
versions of the spec. As well as some potential bugs found, such as using
an same-domain iframe with an email in it, and then rewriting links
therein to do target=_blank, and it was suddenly blocked. They had to open
up frame-src: * in order for the links to open. From my cursory reading of
the spec, it does seem like this is in fact intended behaviour, but I'm
not sure.

The biggest problem however is the interference of pages' CSP policies
when an extension goes mucking around the page doing whatever it likes to

This is not the same as having a CSP-profile on the extension, as Chrome
is doing, but the other way around:

> Extensions can
> inject arbitrary javascript, css into the page and modify the DOM in any
> way. Depending on the CSP policy, those will potentially be blocked. The
> most annoying thing is that those might break your extension or the page
> in subtle ways because some things the extension does work (DOM
> manipulations), but other things fail (scripts/css injection).
> Additionally changes that do fail will generate heaps of false positive
> feedback reports, making the reporting feature a pain to sift through
> and work out "now is this a problem with my CSP poilicy, or is it some
> extension the users installed that's trying to modify the page in some
> way".
> I don't see any realistic solution to this. You'd have to track a whole
> bunch of manipulations and changes to the DOM as either "done by the
> page" or "done by an extension" to work out if they should be allowed or
> not.
> So at first I thought "what a great idea", but after two days of messing
> around and actually trying to use it, I decided that it might be
> bordering on unuseable in the real world

I have not looked into it myself, but this is a very valid concern if we
were to implement it in Opera. What have you that have implemented this
already done about it? How does it work? Is really extensions crippled in
such a way, do they have to think about it?

Odin Hørthe Omdal (Velmont/odinho) · Core, Opera Software, http://opera.com
Received on Wednesday, 29 August 2012 11:35:03 UTC

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