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Date: Tue, 25 Feb 2020 00:54:50 +0500
From: "Alexander E. Patrakov" <patrakov@...il.com>
To: oss-security@...ts.openwall.com
Subject: Re: LPE and RCE in OpenSMTPD's default install (CVE-2020-8794)
On Mon, Feb 24, 2020 at 10:55 PM Qualys Security Advisory
<qsa@...lys.com> wrote:
>
>
> Qualys Security Advisory
>
> LPE and RCE in OpenSMTPD's default install (CVE-2020-8794)
>
>
> ==============================================================================
> Contents
> ==============================================================================
>
> Summary
> Analysis
> ...
> Acknowledgments
>
>
> ==============================================================================
> Summary
> ==============================================================================
>
> We discovered a vulnerability in OpenSMTPD, OpenBSD's mail server. This
> vulnerability, an out-of-bounds read introduced in December 2015 (commit
> 80c6a60c, "when peer outputs a multi-line response ..."), is exploitable
> remotely and leads to the execution of arbitrary shell commands: either
> as root, after May 2018 (commit a8e22235, "switch smtpd to new
> grammar"); or as any non-root user, before May 2018.
>
> Because this vulnerability resides in OpenSMTPD's client-side code
> (which delivers mail to remote SMTP servers), we must consider two
> different scenarios:
>
> - Client-side exploitation: This vulnerability is remotely exploitable
> in OpenSMTPD's (and hence OpenBSD's) default configuration. Although
> OpenSMTPD listens on localhost only, by default, it does accept mail
> from local users and delivers it to remote servers. If such a remote
> server is controlled by an attacker (either because it is malicious or
> compromised, or because of a man-in-the-middle, DNS, or BGP attack --
> SMTP is not TLS-encrypted by default), then the attacker can execute
> arbitrary shell commands on the vulnerable OpenSMTPD installation.
>
> - Server-side exploitation: First, the attacker must connect to the
> OpenSMTPD server (which accepts external mail) and send a mail that
> creates a bounce. Next, when OpenSMTPD connects back to their mail
> server to deliver this bounce, the attacker can exploit OpenSMTPD's
> client-side vulnerability. Last, for their shell commands to be
> executed, the attacker must (to the best of our knowledge) crash
> OpenSMTPD and wait until it is restarted (either manually by an
> administrator, or automatically by a system update or reboot).
>
> We developed a simple exploit for this vulnerability and successfully
> tested it against OpenBSD 6.6 (the current release), OpenBSD 5.9 (the
> first vulnerable release), Debian 10 (stable), Debian 11 (testing), and
> Fedora 31. At OpenBSD's request, and to give OpenSMTPD's users a chance
> to patch their systems, we are withholding the exploitation details and
> code until Wednesday, February 26, 2020.
>
> Last-minute note: we tested our exploit against the recent changes in
> OpenSMTPD 6.6.3p1, and our results are: if the "mbox" method is used for
> local delivery (the default in OpenBSD -current), then arbitrary command
> execution as root is still possible; otherwise (if the "maildir" method
> is used, for example), arbitrary command execution as any non-root user
> is possible.
I would like a bit of clarification. We use OpenSMTPD as a dumb thing
that only relays mail to a central server and never delivers it
locally. The remote server is under our control.
=============
table credentials { smarthost.example.com=myuser:mypassword }
listen on 127.0.0.1
# No local mailboxes
action to_postfix relay host
smtp+tls://smarthost.example.com@...rthost.example.com auth
<credentials> helo myhostname.example.com
match from local for any action to_postfix
=============
Is the hole exploitable in this configuration?
--
Alexander E. Patrakov
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