Treat input as hex values

Question

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?

Answer 1

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

Answer 2

The \x notation is only for string literals in the code itself, not things typed by the user. If you want character \x04, then type ♦ directly into the string.