TwentyOne

While the endGame function is defined as internal, meaning it cannot be called directly by an external user, there are still potential vulnerabilities related to access control. The lack of explicit access restrictions on other functions (such as startGame, hit, call) could still allow attackers or malicious users to manipulate the game or withdraw ether in unauthorized ways.

An attack scenario demonstrating the lack of access control would be as follows:

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An attacker calls the startGame function on behalf of another player or invokes the hit function to manipulate the game, even if they're not the rightful player.

Even though the endGame function is internal, the absence of ownership checks allows a malicious actor to potentially exploit the lack of checks on game interaction to confuse or interfere with the game's logic.

Here's a test case that highlights an abuse scenario:

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_<details>

<summary>Code</summary>

```javascript

// Test Case for Unauthorized Game Manipulation

​

function testUnauthorizedStartGame() public {

address attacker = address(0x123); // Attacker's address

twentyOne = new TwentyOne();

​

// Attempt to start the game without being the player

vm.deal(attacker, 5 ether); // Fund attacker with some ether

​

vm.prank(attacker); // Pretend to be the attacker

twentyOne.startGame{value: 1 ether}(); // This function could be abused by anyone

}```

</details>

​

Since startGame, hit, and call are not properly secured, an attacker could interact with these functions inappropriately, affecting the game.

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The startGame, hit, and call functions are callable by anyone, which could allow any user (even non-players) to interact with the game.

No Ownership or Admin Role: There is no owner or admin role restricting critical functions like endGame, even if it is internal. This means a player could potentially invoke the game logic, but without proper controls, an attacker could exploit other vulnerabilities like resource exhaustion or manipulation.

Impact: Although the internal nature of the endGame function limits direct unauthorized access, the absence of control over the broader game logic and the lack of ownership or privileged access to sensitive actions (such as game initiation, hitting, or calling) exposes the contract to potential issues:

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Unrestricted Access to Game Actions: Players or even non-players could call functions like hit or startGame, interacting with the game in unexpected ways, manipulating the game state, or causing failures.

Insecure Ether Handling: Without proper restrictions on how ether is handled in relation to players, an attacker could manipulate payouts or perform actions that may bypass intended game outcomes.

Denial of Service (DoS): Functions that are intended to be called in a specific order (like startGame or hit) could be abused to exhaust resources (e.g., running out of gas due to loops) or disrupt the flow of the game.

1. Introduce Ownership or Admin Role (Even for Internal Functions):

Even though endGame is internal, critical game logic should be protected by an ownership model (e.g., using Ownable from OpenZeppelin). This will ensure that sensitive actions (like modifying game states, handling payouts) can only be performed by the intended participants or the owner.

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Example with Ownable:

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<summary>Code</summary>

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+ import "@openzeppelin/contracts/access/Ownable.sol";

​

contract TwentyOne is Ownable {

+ function startGame() public payable onlyOwner {

}

​

2. Restrict Player-Specific Functions:

Ensure that the functions like startGame, hit, and call can only be called by the player who initiated the game. This can be done by adding checks such as:

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<summary>Code</summary>

​

+ modifier onlyPlayer() {

+ require(msg.sender == player, "Only the player can perform this action");

+ _;

+ }

​

+ function startGame() public payable onlyPlayer {

+ }

+ function hit() public onlyPlayer {

+ // Allow only the player to hit

+ }

​

​

3. Explicit Access Control in Critical Functions:

Even though endGame is internal, ensure that the logic within functions like startGame, hit, and call is restricted. For example, a require statement can enforce that only the player or a specific role can interact with the game in these ways.

​

4. Audit Game Flow Logic:

Ensure that all actions that can affect the game state (e.g., card drawing, game end, payout logic) are tightly controlled and that no unauthorized entities can interfere with the process.