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Date: Fri, 14 Dec 2012 10:06:43 +0100
From: Florian Weimer <fweimer@...hat.com>
To: oss-security@...ts.openwall.com
Subject: Re: Robust XML validation

On 12/13/2012 01:47 PM, Timo Warns wrote:

>> I wonder if we should care about this in the sense that we should
>> prepare fixes, or if it is sufficient to recommend to validate against
>> trusted schemas/DTDs only.  (I've found an implementation which gets
>> right the things I tested so far, so efficient implementations aren't
>> impossible.)
>
> Validating against trusted schemas/DTDs would not be sufficient in my
> opinion. For example, such validations are not effective against the
> billion laughs attack (http://en.wikipedia.org/wiki/Billion_laughs).

True, entity expansion is required for XML parsing, strictly speaking, 
not just for validation.  Some XML implementations use heuristics to 
stop such attacks.  And of course, there's the big hammer of disallowing 
all entity declarations.

> Moreover, some projects deliberately decide against schema validation.
> For example, when fixing CVE-2012-2665, LibreOffice developers have
> decided against validating the manifest.xml against a schema or DTD.
> If I understood correctly, the reason was that omitting validations
> allows to open documents in a future format on a best-effort basis (as
> an alternative to annoying the user with a "format not supported" message).

I'm not an expert on schema authoring.  (Actually, I once tried to 
define an extensible XML schema and couldn't get it work.) Looking at 
RELAX NG, there doesn't seem to be a way to say, "you can put any tag 
here, but if its <myimportanttag>, it must have *this* structure, 
either.  So it's probably feasible to validate during generation only, 
to check that your hand-crafted code produces the expected document 
structure.  Which is a bit odd.

-- 
Florian Weimer / Red Hat Product Security Team

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