Re: CVE Request: libpam-sshauth: local root privilege escalation

[Vagrant Cascadian <vagrant () debian org>]

On 2016-05-03, Salvatore Bonaccorso wrote:
> On Sun, May 01, 2016 at 10:02:15AM -0400, cve-assign () mitre org wrote:
> > > Due to a programming error, libpam-sshauth returned PAM_SUCCESS where
> > > it should fail with PAM_AUTH_ERR. This was fixed in Debian in the last
> > > upload to unstable with the attached patch.
> > >
> > > https://bazaar.launchpad.net/~ltsp-upstream/ltsp/libpam-sshauth/revision/114
> >
> > We can assign a CVE ID because it appears that something definitely is
> > wrong from the Debian perspective, either the code itself or
> > documentation/lack-of-documentation about how the code was supposed to
> > be used.
> >
> > Use CVE-2016-4422.
>
> Thanks for assigning the CVE identifier.
>
> > However, we don't completely understand the issue:
> >
> > > Introduced with:
> > > https://bazaar.launchpad.net/~ltsp-upstream/ltsp/libpam-sshauth/revision/93/src/pam_sshauth.c
> >
> > Here, the commit message for revision 93 was "Succeed for system
> > accounts."
> >
> > We don't know why introducing the undocumented behavior of "Is it a
> > system user? Fail" would be better than simply not checking
> > "pwent->pw_uid < UID_MIN" at all. Also, is there any risk that, with
> > this libpam-sshauth update, a system's PAM configuration might
> > suddenly provide no way for root to login via SSH?
> >
> > Is it possible that the original motivation for revision 93 was that
> > the PAM_SUCCESS from pam_sm_authenticate was supposed to be specially
> > handled elsewhere in the "pwent->pw_uid < UID_MIN" case?
> >
> > Although not directly applicable to libpam-sshauth, the examples
> > section of the
> > http://www.linux-pam.org/Linux-PAM-html/sag-pam_succeed_if.html man
> > page shows that a set of rules is sometimes designed with UID_MIN
> > special cases.
>
> It might be right that revision 93 cannot be considred the introducing
> revision for the problem. By following the example as given in the
> README.
>
> https://sources.debian.net/src/libpam-sshauth/0.3.1-1/README/#L75
>
> $ cat /etc/pam.d/testservice 
> auth    required        pam_sshauth.so host=127.0.0.1 nostrict # or wherever
> auth    required        pam_exec.so expose_authtok /usr/bin/ltsp-session
> session required        pam_exec.so /usr/bin/ltsp-session
> $ pamtester -v testservice root authenticate open_session close_session
> pamtester: invoking pam_start(testservice, root, ...)
> pamtester: performing operation - authenticate
> Password: <anypassword>
> pamtester: successfully authenticated
> pamtester: performing operation - open_session
> pamtester: successfully opened a session
> pamtester: performing operation - close_session
> pamtester: session has successfully been closed.
>
> I want though to add the Debian maintainer for libpam-sshauth to more
> accurately answer the raised questions, Vagrant Cascadian
> <vagrant () debian org>. 

Also bringing the primary upstream developer, Scott Balneaves
<sbalneav () ltsp org> into the conversation, who has better understanding
of the code.

For this issue, I've largely just discovered it and made some small
effort to backport the patch.


live well,
  vagrant

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