Summary
The startGame() function in the TwentyOne.sol contract returns the player’s initial hand value as a uint256. This allows attackers to manipulate gameplay by reverting unfavorable transactions, ensuring they proceed only with advantageous hands. By using a custom attack contract, an attacker can drain the contract's funds with minimal loss.

Vulnerability Details
Root Cause

The vulnerability lies in the startGame() function, which exposes sensitive game state information (the player's initial hand value) as its return value. This design flaw enables attackers to evaluate the returned value and selectively revert transactions for unfavorable outcomes.

Exploitation Process

1. Initialization:
   The attacker deploys and funds a malicious contract, AttackTwentyOne.sol, which interacts with the TwentyOne.sol contract.

2. Selective Execution:
   The attack contract calls startGame(), checks the returned hand value (playerHand), and reverts if the value is below a predefined threshold (e.g., 20).

3. Guaranteed Advantage:
   Transactions only proceed if the attacker’s hand value is sufficiently high, ensuring a disproportionately high winning probability.

4. Drain Contract:
   The attacker continues exploiting until the target contract’s balance is depleted.

Vulnerable Code

The issue is in the startGame() function of TwentyOne.sol:

Impact
Exploitation Results

1. Win Rate: The attack enables the attacker to win with an artificially high probability.

2. Drain Speed: The contract balance can be drained in approximately 1,500–2,000 calls, depending on the balance and win conditions.

3. Gas Costs: The attack incurs approximately 1,339,241 gas units per reverted transaction, which is minimal compared to the contract's drained value.

Tools Used

1. Solidity Programming: To analyze and manipulate the contract logic.

2. Foundry Framework: For contract deployment, testing, and debugging.

3. Python Script: To automate repeated calls for exploiting the vulnerability.

Proof of Code

Attack Contract (AttackTwentyOne.sol)

Testing Process

1. Deployment:

   • TwentyOne.sol deployed with an initial balance of 20 ETH.

   • AttackTwentyOne.sol deployed with an additional 2 ETH for gas and gameplay funding.

2. Execution:

   • Repeatedly called callTostartGameAndCall() using the attack contract.

   • Observed selective transaction reverts for unfavorable outcomes.

   • Contract balance depleted after ~1,800 calls.

Recommendation

Proposed Fix

The startGame() function should not return the player’s initial hand value, preventing attackers from accessing game state information during the initialization phase.

Fixed Code:

Impact of Fix

Removing the return value eliminates the ability to selectively revert transactions based on the initial hand, closing the exploit vector.

Conclusion

This report demonstrates a critical vulnerability in TwentyOne.sol that enables an attacker to manipulate gameplay and drain the contract’s balance. The proposed fix mitigates this issue by removing sensitive game state exposure during the startGame() phase. Immediate action is recommended to deploy the fix and secure funds in the contract.