Microcorruption - Montevideo#

Microcorruption gives us a debugger and a password to unlock a device, our job is to find the password reading through assembly code and using some reverse engineering tricks. Each challenge is named after a city, each one more difficult than the previous one.

If you didn’t solve the challenge for yourself, I recommend that you stop reading, solve the challenge and check my solution afterwards, do not spoil the challenge and have fun.

How to solve it#

The solution for this problem is almost identical to the Whitehorse, so check that one before check this one.

So, the answer is the same, but the catch is one difference. The input that we insert is being copied from one place of the memory to another place in memory.

Our input is being registered between 0x2400 and 0x242f. The copy of our input is being copied between 0x43ee and 0x441d.

The problem is, the function that copies our input (<strcpy>) halts if a zero-byte is found. So, in the input from Whitehorse, the value was:

666666666666666666666666666666664c450000ff00

But we cannot use the zeroes here, or the value ff00 will not be copied. Let’s replace the zeroes with more 6’s.

666666666666666666666666666666664c456666ff00

With that, the door will be unlocked.