Eggstravaganza
First Flight #37
Beginner FriendlySolidity
100 EXP
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Submission Details
Severity: high
Valid

Weak Randomness in `EggHuntGame::searchForEgg()`.

jobmwitah

Summary

Weak Randomness in EggHuntGame::searchForEgg().

Vulnerability Details

Weak Randomness in EggHuntGame::searchForEgg(). Participants call this function to search for an egg. The use of keccak256 hash functions on predictable values like block.timestamp, block.number, or similar data, including modulo operations on these values, should be avoided for generating randomness, as they are easily predictable and manipulable.

/// @notice Participants call this function to search for an egg.
/// A pseudo-random number is generated and, if below the threshold, an egg is found.
function searchForEgg() external {
require(gameActive, "Game not active");
require(block.timestamp >= startTime, "Game not started yet");
require(block.timestamp <= endTime, "Game ended");
​
// Pseudo-random number generation (for demonstration purposes only)
@> uint256 random = uint256(
@> keccak256(abi.encodePacked(block.timestamp, block.prevrandao, msg.sender, eggCounter))
) % 100;
​
if (random < eggFindThreshold) {
eggCounter++;
eggsFound[msg.sender] += 1;
eggNFT.mintEgg(msg.sender, eggCounter);
emit EggFound(msg.sender, eggCounter, eggsFound[msg.sender]);
}
}

Impact

The users can manipulate the values, block.timestamp and block.prevrandao, to get a random number less than the eggFindThreshold to mint themselves NFTs.

Tools Used

  • Manual review

  • Aderyn static analyzer

Recommendations

The PREVRANDAO opcode also should not be used as a source of randomness. Instead, utilize Chainlink VRF for cryptographically secure and provably random values to ensure protocol integrity.

Updates

Lead Judging Commences

m3dython Lead Judge 7 months ago
Submission Judgement Published
Validated
Assigned finding tags:

Insecure Randomness

Insecure methods to generate pseudo-random numbers

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