I was not able to solve it in the remaining time during the CTF. 

Found this, (obviously not mine):https://github.com/cstanfill/accounting-writeup Original writeup. Contains the original source for the problem.
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It was initially a bit hard to understand, so rewrote it in order to better understand it and step through the code.
</span><span>I posted it here: https://github.com/Caesurus/bkp2017_accounting_simulator/blob/master/exploit.py</span>