I am trying to learn how to create shellcode and I need to input a whole bunch of hex codes. However, when I give give my program an input with hex codes, the hex codes are treated as normal ASCII chars and the backslash is simply just stripped.
Example:
./a.out "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\x35\x85\x04\x08"
In this scenario, the hex codes (\x35\x85\x04\x08) is treated as individual characters - i.e. x=0x75 etc.
The way my program handles input is similar to the following:
int authentication(char *pass){
char password_buffer[16];
strcpy(password_buffer, pass);
return 1;
}
int main(int argc, char *argv[]){
if(authentication(argv[1])){
//access
}
return 1;
}
A dump from memory after strcpy():
0xbffff260: 0x41414141 0x41414141 0x41414141 0x41414141
0xbffff270: 0x78353378 0x30783538 0x38307834 0x08048400
As you can see, at address 0xbffff270 the memory value is 0x78353378, which corresponds to x35x and thus the three first letters in the hex-code input. I would like these four bytes to be 0x08048535.
How do I format my input to do this?
You could use shell printf function like this:
./a.out "$(printf "AAAAA\x08\x32\x20\xfa\x10\x16")"
Hex values will be converted to "binary".
This part is perhaps only adding clutter, and I can and might remove it:
Else you could also do a quick conversion inside your program, to make it easier to use across platforms and systems. A crude sample. (This takes a string and converts any hex sequences where they are presented as \xNN, and in addition prints a hex dump at completion):
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
/*
* Hex Dump
* */
static void txt(unsigned char *buf, int len)
{
int i;
fprintf(stdout, " ");
for (i = 0; i < len; ++i) {
fprintf(stdout,
"%c",
isprint(buf[i]) ?
buf[i] :
'.'
);
}
}
static void xxd(unsigned char *buf, int len)
{
int i, j;
for (i = 0; i < len; ) {
if (!(i % 16)) {
fprintf(stdout,
"%s%03x: ",
i ? "\n" : "",
i
);
}
fprintf(stdout, "%02x ", buf[i]);
if (!(++i % 16))
txt(&buf[i - 16], 16);
}
if ((i % 16)) {
for (j = i; j % 16; ++j)
fprintf(stdout, " ");
txt(&buf[i - (i % 16)], i % 16);
}
fprintf(stdout, "\n");
}
/*
* Hex char to value.
* */
static unsigned hexval(char c)
{
if (c >= '0' && c <= '9')
return c - '0';
if (c >= 'a' && c <= 'f')
return c - 'a' + 10;
if (c >= 'A' && c <= 'F')
return c - 'A' + 10;
return ~0;
}
/*
* Convert all hex sequences.
* */
int str2bin(char *data, unsigned char **buf_ptr)
{
int len, converted = 0;
unsigned int val;
unsigned char *buf;
buf = malloc(strlen(data) + 1);
*buf_ptr = buf;
for (; *data; ) {
/* If next char is not backslash copy it and continue */
if (*data != '\\') {
*buf++ = *data++;
continue;
}
/* If we have anything else then x or X after, return error. */
if (data[1] != 'x' && data[1] != 'X')
return -1;
val = (hexval(data[2]) << 4) | hexval(data[3]);
/* If not valid hex, return error. */
if (val & ~0xff)
return -1;
*buf++ = val;
data += 4;
/* Keep track of converted numbers, "for fun". */
++converted;
}
len = buf - *buf_ptr;
fprintf(stderr, "%d hex values converted.\n", converted);
return len;
}
int main(int argc, char *argv[])
{
int len;
unsigned char *buf = NULL;
if (argc < 2) {
fprintf(stderr, "Missing argument.\n");
return 1;
}
if ((len = str2bin(argv[1], &buf)) < 0) {
fprintf(stderr, "Bad input.\n");
return 2;
}
xxd(buf, len);
free(buf);
return 0;
}
Giving you something like this:
$ ./hexin "Hello\x20\x44\x65\x08\x01\x00\x00\xf7\xdf\x00\x02Bye"
11 hex values converted.
000: 48 65 6c 6c 6f 20 44 65 08 01 00 00 f7 df 00 02 Hello De........
010: 42 79 65 Bye