Date: Tue, 3 May 2016 14:05:36 -0500 From: Scott Balneaves <sbalneav@...p.org> To: Vagrant Cascadian <vagrant@...ian.org> Cc: Salvatore Bonaccorso <carnil@...ian.org>, cve-assign@...re.org, oss-security@...ts.openwall.com Subject: Re: CVE Request: libpam-sshauth: local root privilege escalation >> 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? The problem was, quite bluntly, an incomplete understanding of PAM mechanics on my part. The original idea was that it was supposed to be used in conjunction with other modules; specifically, pam_unix. So my *thinking* (if you could call it that) was that it would be used as such: auth required pam_unix.so ... auth required pam_sshauth.so ... Since (in my mind), accepting the root user would be handled by pam_unix, I should simply succeed, since if the root account password was incorrectly entered, the pam_unix result would be a FAIL, and thus then entire pam auth stack would fail. Therefore, in my (incorrect) thinking, I should simply succeed on systems accounts. I didn't, at the time, know about the ability to skip with [success=N], or even consider that I would use it as the only pam module. TL;DR: I didn't know what I was doing, and misunderstood how I should handle systems accounts. Mea culpa, mea maxima culpa. Cheers, Scott On Tue, May 3, 2016 at 1:51 PM, Vagrant Cascadian <vagrant@...ian.org> wrote: > On 2016-05-03, Salvatore Bonaccorso wrote: > > On Sun, May 01, 2016 at 10:02:15AM -0400, cve-assign@...re.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@...ian.org>. > > Also bringing the primary upstream developer, Scott Balneaves > <sbalneav@...p.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 >
Powered by blists - more mailing lists
Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.