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Re: Robust XML validation
From: Timo Warns <warns () pre-sense de>
Date: Thu, 13 Dec 2012 13:47:15 +0100

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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