oss-security - Re: Re: expat bug 1990430

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Date: Thu, 27 Aug 2009 09:27:29 +0100
From: Joe Orton <jorton@...hat.com>
To: "Steven M. Christey" <coley@...us.mitre.org>
Cc: oss-security@...ts.openwall.com, Robert Buchholz <rbu@...too.org>
Subject: Re: Re: expat bug 1990430

On Wed, Aug 26, 2009 at 12:38:26PM -0400, Steven M. Christey wrote:
> 1) neon "when expat is used" was subject to the billion laughs attack
>    (recursion during entity expansion).  This was assigned CVE-2009-2473.
>    The description for CVE-2009-2473 focuses on neon, and I haven't seen
>    it used for other products.  Was this really a problem in expat?  Then
>    we may have a dupe.

I can't find the reference now, but, the expat maintainers said 
something like this:

a) it's broadly expected behaviour that certain well-formed XML 
documents can cause consumption of CPU/memory in the XML parser which is 
disproportionate to the size of the input document.

b) this is a deliberate design decision for expat; it is not possible to 
mitigate CPU/memory consumption attacks *by default* in the XML parser 
without having expat reject some well-formed XML documents.

I don't think that is an unreasonable position, especially if you 
consider that an XML parser exists primarily for document processing, 
rather than for decoding a network protocol.

Any code using expat which parses XML documents from an untrusted source 
will therefore be vulnerable to CPU/memory consumption attacks, unless 
that code takes specific steps to mitigate those attacks.  The only 
attack I'm aware of is the "billion laughs" attack, which can be 
mitigated using a couple of expat API calls to disable entity expansion. 
[1]

So, on this basis we've treated "use of expat API to parse XML from 
untrusted sources without taking steps to mitigate billion laughs 
attack" as a vulnerability in the code *using* the expat API; rather 
than in expat itself.  For apr-util this vulnerability was 
CVE-2009-1955, for neon, it was CVE-2009-2473.

Hope this makes sense.

Regards, Joe

[1] It is perhaps worth noting that the WebDAV RFC in fact highlights 
the need for implementors to configure XML parsers such as to avoid 
attacks, and indeed this specific attack:

http://tools.ietf.org/html/rfc4918#section-20.6

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