W3C home > Mailing lists > Public > public-webappsec@w3.org > February 2016

Re: In-browser sanitization first, "Safe Node" later?

From: Craig Francis <craig.francis@gmail.com>
Date: Mon, 8 Feb 2016 09:34:57 +0000
Cc: "public-webappsec@w3.org" <public-webappsec@w3.org>
Message-Id: <8A95A185-B565-402B-A041-EE7FE23256D1@gmail.com>
To: Frederik Braun <fbraun@mozilla.com>
As a web developer who frequently has to sanitise HTML (more so server side), I would still like to see this.

But creating a safe node list will be difficult.

Take the <a> as an example, imagine a forum with a WYSIWYG (/me shudders)... some forums won't like this at all (SEO spamming), some may consider this safe if it has a rel=nofollow... but many will forget the href="javascript:...", which is a valid attribute on a valid node, but getting a click event can cause inline JavaScript to run (assuming no CSP that blocks unsafe-inline).

If you know how to solve this (both as a Sanitiser or under a Safe Node), then I'll be very happy.

Craig



> On 8 Feb 2016, at 08:48, Frederik Braun <fbraun@mozilla.com> wrote:
> 
> Hi,
> 
> I think there is a need for a client-side HTML/XSS sanitization
> mechanism that lives in the browser (i.e., where the parser is).
> AFAIU, previous discussion has shown that there are no strong objections
> to this, but feel free to look the previous thread [1] or Mario
> Heiderich's presentation from Usenix Enigma [2] for further reading.
> 
> I think that a first version of this spec should be a JavaScript API
> that consumes a string of potentially dangerous markup and returns a
> string that is clean.
> 
> A Safe Node is certainly more interesting, but I'm afraid that we (the
> working group) are sometimes too detached from the needs of a modern web
> application and that we should start with providing something useful *soon*.
> As we have seen with CSP, it's always harder to retrofit a new security
> system to an existing architecture. But the "String In - String Out"
> approach will certainly fit into every app. We can still do the Safe
> Node in a follow-up, if the initial feedback is good.
> 
> Another outcome of the reduced first version would be a public, vetted
> and testable whitelist of safe DOM Nodes. This is useful for all
> existing custom sanitizers and is a positive outcome of its own [3].
> 
> I expect that this first version will be easy to implement, given that
> existing browsers use already this internally, albeit not exposed to web
> content.
> 
> In the long run, attackers might race towards finding and abusing parser
> bugs and more quirks like those which Mario has called mXSS (mutation
> XSS) [4]. This is good, as it will guide us to what a Safe Node will
> need and prove that we have indeed risen the bar beyond trivial XSS
> exploits.
> 
> Thoughts?
> 
> 
> Cheers,
> Frederik
> 
> 
> [1] For the initial thread "In-browser sanitization vs. a “Safe Node” in
> the DOM" see
> https://lists.w3.org/Archives/Public/public-webappsec/2016Jan/thread.html
> 
> [2] Link to his slides and the abstract at
> https://www.usenix.org/conference/enigma2016/conference-program/presentation/heiderich
> 
> [3] Obsolete whitelist at WHATWG wiki:
> https://wiki.whatwg.org/wiki/Sanitization_rules
> 
> [4] "mXSS Attacks: Attacking well-secured Web-Applications
> by using innerHTML Mutations", see https://cure53.de/fp170.pdf
> 
Received on Monday, 8 February 2016 09:35:31 UTC

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