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Message-ID: <20111223232211.GA20465@openwall.com>
Date: Sat, 24 Dec 2011 03:22:11 +0400
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Cc: Jeff Mitchell <mitchell@....org>, cve@...re.org
Subject: Re: Disputing CVE-2011-4122
On Wed, Dec 07, 2011 at 09:26:44AM -0500, Jeff Mitchell wrote:
> I've been asked by the kcheckpass maintainer to lodge a dispute of
> CVE-2011-4122.
>
> As explained in the blog entry linked from the CVE[1], the problem is
> that neither kcheckpass nor OpenPAM validate the 'service_name' input
> argument of pam_start(). This hole can be used to make PAM load
> arbitrary shared libraries, which can be used to execute arbitrary code
> as root, as kcheckpass is setuid root.
Besides loading of arbitrary shared libraries due to the OpenPAM
peculiarity, doesn't this kcheckpass feature allow a user to specify an
arbitrary PAM service name that is actually valid on the system, not
limited to PAM services intended for use by KDE? Isn't that a
vulnerability of itself? While kcheckpass itself doesn't grant access
to any resource, an alternate PAM stack (maybe only intended for use by
a specific service) might have modules with side-effects.
To me, kcheckpass looks poorly designed in letting the invoking user
specify an arbitrary PAM service name. Instead, the service name should
either be obtained from a trusted place or it should be checked against
a trusted whitelist.
For example, passwd(1) from SimplePAMApps does the latter: it has a
command-line option to specify a PAM service name suffix (which it'd
append to "passwd"), but the suffix must be whitelisted in
/etc/security/passwd.conf.
> One could assume that kcheckpass should do the validation. However, the
> PAM documentation makes no mention of what a service name is supposed to
> look like, and consequently it must be treated as opaque by the
> application code. Therefore all validation must be expected to be done
> by the library, and failure to do so must be seen as a bug in the
> library exclusively.
These are some valid points, but on the other hand giving an untrusted
user full control over the PAM service name is so ridiculous that I
can't blame a PAM library exclusively for not sanitizing this input.
I think we'd set a bad precedent by saying that kcheckpass did
everything right and only OpenPAM was at fault. I think that both need
to be corrected. OpenPAM is already patched by now; is kcheckpass
corrected in some way as well?
http://security.freebsd.org/advisories/FreeBSD-SA-11:10.pam.asc
> As a result, it is correct to list kcheckpass as an affected
> application, but not as the origin of the vulnerability.
My opinion is that the vulnerability originates in both places, or we
can say that there's more than one vulnerability here. If I had to
choose just one, I'd say that kcheckpass is more at fault than OpenPAM,
and that OpenPAM's fix is a hardening measure, whereas kcheckpass needs
to have its vulnerability fixed.
I am not affiliated with either project; in fact, I am not even using
KDE and OpenPAM normally.
I am of a similar opinion regarding other cases where poorly written or
misconfigured applications expose library interfaces that were never
meant to be used on untrusted input. For example, the "glibc: timezone
integer overflow" issue that was brought to this list earlier this month
is in my opinion less of an issue than FTP server misconfiguration that
makes the issue exploitable. That is, I'd rather assign CVE ids to FTP
servers supporting chroot'ing into users' home directories without
warning the server admins of the added root compromise risk inherent to
such chroot'ing. Patching one attack vector in glibc does not eliminate
that risk.
http://www.openwall.com/lists/oss-security/2011/12/05/1
Alexander
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