Mystery Box

Summary

Weak randomness due to a modulo on block.timestamp in function openBox. These can be influenced by miners to some extent so this should be avoided.

Vulnerability Details

Using block.timestamp as a source of randomness is commonly advised against, as the outcome can be manipulated by calling function openBox. In this case, a compromised function openBox retries the selection of the coins until the result is favorable to the user.

Let`s guess Tom is a miner. Tom calls the function openBox and re-orders the block containing the transaction. As a result, Tom can own a lot of Gold Coins and abuses the contract.

Impact

Let`s guess that Tom is a miner. Tom calls the function openBox and re-orders the block containing the transaction. As a result, Tom can own a lot of Gold Coins and abuses the contract.

Profit from this is that Tom buys a Box for 0.1 ETH and wins the highest reward - a Gold Coin that costs 1 ETH. Profit = 0.9 ETH (1 - 0.1).

Tools Used

Manual review.

Recommendations

Do not use block.timestamp as a source of randomness.