Intro to Whitebox Pentesting: Blind Exploitation

For this section I need to do the following:
Challenge: Try to use what you learned in this section to reached boolean-based exfiltration using the exercise from the previous section, in which this would be possible. Instead of sleeping, you may send a different HTTP response code (e.g. 200 for match and 404 for fail). This would make you thoroughly understand how both techniques work, and how they differ from each other.

The flag at ‘/flag.txt’ consists of 3 digits. Try to use the payload from this section to find them, by iterating over [0-9] for each of the 3 digits (max 30 attempts needed)

Can someone help to solve this? This is what I have done so far: PrivateBin

This issues has been resolved by my self.