TwentyOne

Summary

In the dealersHand() function of the TwentyOne contract, Kings are incorrectly valued as 0 instead of 10. This miscalculation disrupts the dealer's hand totals, affecting the game's fairness and balance.

Vulnerability Details

• Issue: Kings are represented by cardValue == 0 after the modulo operation (cardValue = card % 13).

• Problematic Code:

  if (cardValue >= 10) {

  dealerTotal += 10;

  } else {

  dealerTotal += cardValue;

  }

• Explanation: The condition cardValue >= 10 does not capture Kings (cardValue == 0), causing them to add 0 to dealerTotal instead of 10.

Impact

• Altered Dealer Hand Totals: Dealer's hand totals are inaccurately low when Kings are drawn.

• Disrupted Game Balance: The fairness of the game is affected, potentially giving an unintended advantage.

• Deviation from Rules: The game deviates from standard blackjack rules, affecting player trust.

Tools Used

• Manual code review of the dealersHand() function.

Recommendations

• Correct King Valuation:

  if (cardValue == 0 || cardValue >= 10) {

  dealerTotal += 10;

  } else {

  dealerTotal += cardValue;

  }

• Ensure Consistency: Verify that all face cards are correctly valued in all functions.

• Implement Tests: Add unit tests to confirm that Kings are valued at 10.