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General Skills 400: Store

Challenge

We started a little store, can you buy the flag? Source. Connect with 2018shell1.picoctf.com 5795.

Solution

Playing around with a local copy, trying to buy 1000000000 imitation flags
overflows and wraps around, giving you a very positive balance. Then you can
just buy the real flag.

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Welcome to the Store App V1.0
World's Most Secure Purchasing App
[1] Check Account Balance
[2] Buy Stuff
[3] Exit
 Enter a menu selection
2
Current Auctions1
[1] I Can't Believe its not a Flag!
[2] Real Flag
Imitation Flags cost 1000 each, how many would you like?
1000000000
Your total cost is: -727379968
Your new balance: 727381068
Welcome to the Store App V1.0
World's Most Secure Purchasing App
[1] Check Account Balance
[2] Buy Stuff
[3] Exit
 Enter a menu selection
2
Current Auctions
[1] I Can't Believe its not a Flag!
[2] Real Flag
2
A genuine Flag costs 100000 dollars, and we only have 1 in stock
Enter 1 to purchase1
YOUR FLAG IS: picoCTF{numb3r3_4r3nt_s4f3_dbd42a50}

Flag

picoCTF{numb3r3_4r3nt_s4f3_dbd42a50}