smachine

Challenge Overview

Name: smachine

Author: @acez

Description: Sometimes reversing is just blackbox testing.

Flag format: midnight{...}

Objective: Interact with the blackbox Simple Machine service, discover the available operations, and set a register to the required winning value.

Files Provided

Solution Plan

1. Connect to the remote service and identify the available commands.

2. Use help, regs, and win to understand the machine state and win condition.

3. Set any register to 0x1337 using allowed arithmetic and bitwise operations, then run win.

Code (Exploit Script)

nc smachine.play.ctf.se 9189

help
regs
win

add x9 65535
and x9 70455
regs
win

Flag

midnight{s1MpL3_sT4cK_M4cH1N3}

Notes

The service starts a REPL named Simple Machine. The help command lists the supported operations:

add  and  help  mul  or  regs  sub  win  xor

The win command reveals the condition:

Set any register to 0x1337 in order to win.

Directly adding 4919 (0x1337) returned Bad Result, but bitwise operations allowed reaching the target value. Setting x9 to 0xffff and applying and x9 70455 results in 0x1337, which satisfies the win condition.