dc2020q - supersafecalc
This was a jeopardy challenge, part of dc2020q. (CTFtime)
You may spawn the server:
On your laptop
On your laptop (full source)
Hosted by OOORight now we're asking players to login first.
A really safe calculator. The flag is in /flag
Hint 1Start by thinking how you can trigger SIGFPE in a permissible way.
Hint 2It still involves division, but not in the usual way.
Hint 3Can you allocate memory for what doesn't have it?
Hint 4You need to exploit a race condition. It shouldn't take too much time and it does happens with fair reliability, but you may still want to practice on your laptop to ease retrying.
If you wish, you can contribute more.
There's an author-recommended writeup.