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Date: Thu, 13 Dec 2012 13:47:15 +0100
From: Timo Warns <warns@...-sense.de>
To: oss-security@...ts.openwall.com
Subject: Re: Robust XML validation
On 12.12.2012 18:11, Florian Weimer wrote:
> I'm working on guidelines for robust XML parsing and I noticed that
> there are some denial-of-service issues related to validation which do
> not seem widely documented (but were apparently known when SGML was
> specified).
I'm interested in such guidelines. Will they be public?
> 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).
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).
Regards, Timo
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