Fun with Assembly 11: More smashing through the stack

Posted by Hywel Carver on August 26, 2015

This is the eleventh in a series. You might want to read the previous post before reading this.

This post is based on the Montevideo level on microcorruption.com. Like last time, we’re trying to find an input to open a lock without knowing the correct password, using knowledge of assembly language.

A suggestion

Stop reading. This one works very similarly to the previous level (see link above) - go and re-read that, then try really hard to solve this level yourself, before you read anything below. The one thing you might need to know is that strcpy will stop copying when it reaches the first null byte (0x00).

First steps

This looks so similar to the last level (see link above). What’s different? Well this time, the input isn’t directly taken onto the stack. It’s stored at 0x2400.

4508:  3e40 3000      mov   #0x30, r14
450c:  3f40 0024      mov   #0x2400, r15
4510:  b012 a045      call  #0x45a0 <getsn>

It’s then copied to the stack (which again has only 16 bytes added for 48 potential bytes of input).

4514:  3e40 0024      mov   #0x2400, r14
4518:  0f41           mov   sp, r15
451a:  b012 dc45      call  #0x45dc <strcpy>

Then the area where the input was written is reset.

451e:  3d40 6400      mov   #0x64, r13
4522:  0e43           clr   r14
4524:  3f40 0024      mov   #0x2400, r15

The difference this makes is all in the string copying. The strcpy will copy up to the first null byte (0x00) and then stop. That means we can’t have any null bytes in our input string if we want it to work. The address of the INT function has also changed, to 0x454c.

Our answer before was 00000000000000000000000000000000324500007f. Changing for the new address of INT gives us 000000000000000000000000000000004c4500007f. If we now change every 0 to a 4 (to avoid the problematic null bytes), we end up with 444444444444444444444444444444444c4544447f, and…

The door springs open

This is really a variation on a theme. We’re applying ideas you’ve already seen before, with the extra constraint that we can’t have a null byte in the middle of our input.