The writeup will be relatively short as the challenge was quite easy to solve!

Files

ready.exe

Solution:

Analyzing the binary with DIE yielded a UPX 3.96 packed binary. photo_2022-04-16_20-19-21.jpg

So the first thing I did was upx -d ready.exe, which would have been correct if it weren’t for the author confirming it themselves that the unpacking broke the binary (I discovered this only after the CTF ended) During the CTF I noticed that the binary broke, and I did not trust the static analysis of it apart from some more info on symbols, albeit being broken.

I had no other choice to stuck with running the packed binary, which was a pain in the butt since I had no helpful script as the UPX version used was literally the latest one available at the time of writing this :(

I stuck with x64dbg instead of ida since ida’s debugger did not like imports changing at runtime (it did not detect them at all by default and I did not want to search for a fix for that as x64dgb worked well enough)

I noticed that the binary complained about a missing DCTF.dll when run. I supposed this was just to annoy me so I copied the ready.exe binary and renamed to DCTF.dll hoping there were no particular checks if it were actually a valid dll (and I was right)

Passed that and running it again, the program wrote on the console that I “was close” and then stopped itself. Obviously the first thing I did was to put a breakpoint in the wsprintfw symbol, which is the “print” function used in order to check what was actually making it print that (and eventually reverse it why if it were an if clause)

photo_2022-04-16_20-15-53.jpg

Ironically that was enough to solve the challenge as the flag was unpacked from the program itself and put in the memory at that breakpoint time :D (I confirmed with the author that is indeed the intended solution, but statically reversing it was also possibile since the non-null values were just XORred with a key)

photo_2022-04-16_20-15-24.jpg