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Date: Wed, 5 Jun 2013 12:18:58 -0600
From: Vincent Danen <vdanen@...hat.com>
To: oss-security@...ts.openwall.com
Cc: audreyt@...reyt.org
Subject: CVE-2013-2145: perl Module::Signature code execution vulnerability

Florian Weimer of the Red Hat Product Security Team reported the
following flaw (it has been given the name CVE-2013-2145):


The perl Module::Signature module adds signing capabilities to CPAN
modules.  The 'cpansign verify' command will automatically download keys
and use them to check the signature of CPAN packages via the SIGNATURE
file.

The format of the SIGNATURE file includes the cipher to use to match the
provided hash; for instance:

SHA1 955ba924e9cd1bafccb4d6d7bd3be25c3ce8bf75 README

If an attacker were to replace this (SHA1) with a special unknown cipher
(e.g. 'Special') and were to include in the distribution a
'Digest/Special.pm', the code in this perl module would be executed when
'cpansign -verify' is run.  This will execute arbitrary code with the
privileges of the user running cpansign.

Because cpansign will download public keys from a public key repository,
the GPG key used to sign the SIGNATURE file may also be suspect; an
attacker able to modify a CPAN module distribution file and sign the
SIGNATURE file with their own key only has to make their key public.
cpansign will download the attacker's key, validate the SIGNATURE file
as being correctly signed, but will then execute code as noted above, if
the SIGNATURE file is crafted in this way.

Module::Signature version 0.72 corrects [1],[2] this issue by refusing
to load Digest::* modules from relative paths in @INC.

References:

https://github.com/audreyt/module-signature/commit/575f7bd6ba4cc7c92f841e8758f88a131674ebf2
https://github.com/audreyt/module-signature/commit/cbd06b392a73c63159dc5c20ff5b3c8fc88c4896
https://bugzilla.redhat.com/show_bug.cgi?id=971096


As an aside, I don't believe the authenticity checks of cpansign are
valid.  According to the upstream documentation, the defaults are to
download any matching public key for anything that is GPG-signed.  That
by itself, since there cannot be any trust, makes me believe that the
GPG signature is no more useful than an md5sum of the SIGNATURE file (in
terms of it being "tamper proof").  It's sufficient to verify that the
contents of the file are unmodified from whomever signed it, but does
not establish any kind of trust/trustworthiness.

According to:

http://search.cpan.org/~audreyt/Module-Signature-0.71/lib/Module/Signature.pm

the following is documented:

$AutoKeyRetrieve
Whether to automatically fetch unknown keys from the key server.
Defaults to 1.

$KeyServer
The OpenPGP key server for fetching the author's public key (currently
only implemented on gpg, not Crypt::OpenPGP). May be set to a false
value to prevent this module from fetching public keys

In light of the above, I wouldn't consider the untrustworthiness of a
GPG key to be a flaw, but it does essentially make the GPG signature
part not have any real value.

I've suggested to upstream that if they want this to be used seriously
for trust (and not just verifying that the distribution is untampered
with, according to whomever was able to sign the SIGNATURE file), that
they should disable the auto-retrieval of keys by default and/or CPAN
should manage their own keyserver of trusted keys and cpansign should
only pull from that keyserver.  The first is probably practical enough
to do, the second I'm not so sure.

If anyone has any other suggestions for upstream, I've cc'd her to this
mail (she's not on the list AFAIK, so keep her cc'd if you have other
suggestions).

I will offer that this is at least a step in the right direction as I
don't believe that pypi for python does anything like this (no idea for
other languages).

-- 
Vincent Danen / Red Hat Security Response Team 

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