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Date: Mon, 5 May 2014 21:00:51 +0200
From: Tomas Hoger <>
Subject: Re: CVE Request: libxml2 external parsed entities

On Mon, 28 Oct 2013 19:17:51 +0100 Nicolas Grégoire wrote:

> It's still unclear to me what exactly CVE-2013-0339 covers.

I agree that this CVE still remains rather unclear.  The original
assignment that is limited to:

  CVE-2013-0338 - libxml2 internal entity expansion
  CVE-2013-0339 - libxml2 external entities expansion

does not really help clarify what was the original intention of the
CVE, and its current use does not seem consistent with what I believe
the intention was.

> Patch for the entity expansion DoS:

That commit is referenced by CVE-2013-0338, which currently has a
sufficiently clear description of being a XEE DoS issue, really a
billion laughs variant, or even a CVE-2003-1564 incomplete fix.

> Patch for not expanding external entities by default:

That commit is indeed listed as reference for CVE-2013-0339.  Note that
it's not about expanding, but about (not) loading and parsing external
entities when entity expansion is disabled.

The initial assignment just says "libxml2 external entities expansion"
with no other details, which I tend to read as "libxml2 performs
expansion of external entities by default, which leads to the usual XXE

Such assignment seems incorrect, as libxml2 does not expand entities
(internal or external) by default, it only does so when entity
substitution is requested using XML_PARSE_NOENT parser option.

Current CVE description "libxml2 through 2.9.1 does not properly handle
external entities expansion unless an application developer uses the
xmlSAX2ResolveEntity or xmlSetExternalEntityLoader function, ...",
which may mean "when libxml2 is instructed to perform entity expansion,
there's no easy way to make it only expand internal entities and avoid
external entities to avoid XXE issues".

That seems correct, as there is no option to make libxml2 only process
internal entities and raise error when external entity is encountered.
It provides XML_PARSE_NONET parser option to avoid using remote
external entities, but nothing similar to block local external entities
(i.e. issues with reading /etc/passwd, huge file, or fifo file).

However, the above commit does not make a difference here.  It does not
prevent fetching of external entities when parsing with NOENT.  It is
not a fix for the CVE if defined this way.

AFAICS, CVE-2013-0339 description should either be updated to correctly
describe how the id was used (correctly or not) for issue fixed via
4629ee0 commit, or a new id is required for 4629ee0.

> Are both patches covered? The second one is quite important as it
> kills the classic XXE vector <!ENTITY foo SYSTEM "/etc/passwd">

I don't believe 4629ee0 actually makes a difference for this specific
case.  When parsing without NOENT, /etc/passwd was loaded, but foo
entity references were not replaced by the passwd file content.  When
parsing with NOENT, 4629ee0 does not change libxml2 behavior.

Tomas Hoger / Red Hat Security Response Team

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