Comment by 0x457

This exploit depends on the deployment. With PHP-FPM, an OOM during request startup terminates the child, so the corrupted state wouldn’t survive for subsequent requests to exploit. The challenge used PHP’s built-in server, where the worker was reused.

Doesn't mean that there isn't another exploit chain that can be used though. Challenge also had 4 workers total, so pining one worker wasn't that hard.