BroncoCTF : Negative Bread Writeup

Short summary

A CTF writeup for the Negative Bread challenge from BroncoCTF involving a 64-bit ELF binary simulating a bank. The goal is to reach a $1,000,000 balance from $100 to trigger a hidden win() function. The vulnerability is a bounds-check mismatch: the dispute feature validates abs(amount) against the limit but credits the original signed value, allowing a single dispute of $999,999 to push the balance over the threshold and print the flag.

• •ELF binary bank simulator requires reaching $1M balance to trigger win() function
• •Dispute feature checks abs(amount) but credits the original signed value — a bounds-check vs effect mismatch
• •A single dispute of $999,999 starting from $100 clears the threshold with no overflow needed

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