Submission Details
Severity:
selectWinner function can be exploited via MEV
Summary
Raffle loosers can use MEV to determine the next winner and rescue stake.
Vulnerability Details
selectWinner function calculates winner thus:
uint256 winnerIndex =
uint256(keccak256(abi.encodePacked(msg.sender, block.timestamp, block.difficulty))) % players.length;
Using a MEV transaction call, a caller can easily calculate the winner since all the three calculation parameters are known at the time the block is generated. On determining that they have lost, they can easily call refund + selectWinner.
Impact
The contest looses fee from MEV users.
Tools Used
Manual review
Recommendations
Disable refund after contest period has ended. Add this code to refund function:
require(block.timestamp < raffleStartTime + raffleDuration, "PuppyRaffle: Raffle is over");
Updates
Lead Judging Commences
Hamiltonite about 2 years ago
Submission Judgement Published Assigned finding tags:
weak-randomness
Root cause: bad RNG Impact: manipulate winner
selectWinner can be frontran
Rewards breakdown
nSLOC
143This contest has a flat reward structure with the following payouts:
100 EXP
20 EXP
2 EXP
Live
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