diff --git a/13-04-26-test/.gitignore b/13-04-26-test/.gitignore
new file mode 100644
index 00000000..85198aaa
--- /dev/null
+++ b/13-04-26-test/.gitignore
@@ -0,0 +1,14 @@
+# Compiler files
+cache/
+out/
+
+# Ignores development broadcast logs
+!/broadcast
+/broadcast/*/31337/
+/broadcast/**/dry-run/
+
+# Docs
+docs/
+
+# Dotenv file
+.env
diff --git a/13-04-26-test/.gitmodules b/13-04-26-test/.gitmodules
new file mode 100644
index 00000000..888d42dc
--- /dev/null
+++ b/13-04-26-test/.gitmodules
@@ -0,0 +1,3 @@
+[submodule "lib/forge-std"]
+	path = lib/forge-std
+	url = https://github.com/foundry-rs/forge-std
diff --git a/13-04-26-test/README.md b/13-04-26-test/README.md
new file mode 100644
index 00000000..5ea10a5d
--- /dev/null
+++ b/13-04-26-test/README.md
@@ -0,0 +1,106 @@
+# Overview
+
+This project demonstrates a classic **Reentrancy Vulnerability** in Solidity. It includes an exploit contract and a secured version of the contract that implements industry-standard fixes.
+
+---
+
+## Project Structure
+
+* **src/VulnerableContract.sol**: The original contract with the reentrancy bug.
+* **src/Attacker.sol**: Malicious contract used to drain funds.
+* **src/FixedContract.sol**: The secured version using CEI and a Reentrancy Guard.
+* **test/VulnerableContract.t.sol**: Foundry tests that prove the exploit and the fix.
+
+---
+
+## Getting Started
+
+### Prerequisites
+
+Make sure you have [Foundry](https://book.getfoundry.sh/getting-started/installation) installed on your machine.
+
+### Installation & Setup
+
+1. **Clone the repository:**
+
+    ```bash
+    git clone https://github.com/rayeberechi/buidl-test.git
+    cd buidl-test
+    ````
+
+2. **Install dependencies:**
+
+   ```bash
+   forge install
+   ```
+
+3. **Compile the contracts:**
+
+   ```bash
+   forge build
+   ```
+
+---
+
+## How to Run Tests
+
+1. Run all tests to see the exploit and the fix in action:
+
+    ```bash
+    forge test -vv
+    ```
+
+2. To see the step-by-step transaction "trace" of the exploit:
+
+```bash
+forge test --match-test test_ExploitDrainsVulnerable -vvvv
+```
+
+---
+
+## 1. The Vulnerability
+
+In `VulnerableContract.sol`, the `withdraw` function is unsafe because it sends money **before** updating the user's balance.
+
+Because the contract hasn't recorded that the money is gone yet, an attacker can re-enter the function and repeatedly withdraw funds.
+
+---
+
+## 2. The Exploit
+
+The `Attacker.sol` contract exploits this by:
+
+* **Depositing** a small amount of ETH to become a valid user.
+* **Withdrawing** that amount.
+* **The Loop:** When ETH is sent back, it triggers the attacker's `receive()` function.
+* This function immediately calls `withdraw` again before the first execution finishes.
+* This continues recursively until the contract is drained.
+
+---
+
+## 3. The Solution
+
+`FixedContract.sol` uses two layers of protection:
+
+### * Checks-Effects-Interactions (CEI)
+
+* Update the user’s balance **before** sending ETH.
+* Prevents re-entry because balance is already set to 0.
+
+### * Reentrancy Guard
+
+* Uses a `locked` state variable via a `noReentrant` modifier.
+* Prevents the function from being called again while it's still executing.
+
+---
+
+## 4. Test Results
+
+* `test_ExploitDrainsVulnerable`
+  Confirms the attacker successfully drains the vulnerable contract.
+
+* `test_LegitUserCanWithdrawFromFixed`
+  Confirms legitimate users can safely withdraw funds.
+
+* `test_AttackFailsOnFixed`
+  Confirms the exploit is blocked and funds remain secure.
diff --git a/13-04-26-test/foundry.lock b/13-04-26-test/foundry.lock
new file mode 100644
index 00000000..bc06b89b
--- /dev/null
+++ b/13-04-26-test/foundry.lock
@@ -0,0 +1,8 @@
+{
+  "lib/forge-std": {
+    "tag": {
+      "name": "v1.15.0",
+      "rev": "0844d7e1fc5e60d77b68e469bff60265f236c398"
+    }
+  }
+}
\ No newline at end of file
diff --git a/13-04-26-test/foundry.toml b/13-04-26-test/foundry.toml
new file mode 100644
index 00000000..25b918f9
--- /dev/null
+++ b/13-04-26-test/foundry.toml
@@ -0,0 +1,6 @@
+[profile.default]
+src = "src"
+out = "out"
+libs = ["lib"]
+
+# See more config options https://github.com/foundry-rs/foundry/blob/master/crates/config/README.md#all-options
diff --git a/13-04-26-test/question.md b/13-04-26-test/question.md
new file mode 100644
index 00000000..efd0f7d2
--- /dev/null
+++ b/13-04-26-test/question.md
@@ -0,0 +1,30 @@
+### Questions:
+- Identify the vulnerability in this contract.
+- Explain how an attacker can exploit it.
+- Rewrite the withdraw function to fix the issue.
+```
+mapping(address => uint256) public balances;
+
+function deposit() public payable {
+    balances[msg.sender] += msg.value;
+}
+
+function withdraw(uint256 amount) public {
+    require(balances[msg.sender] >= amount, "Insufficient balance");
+
+    (bool success, ) = msg.sender.call{value: amount}("");
+    require(success, "Transfer failed");
+
+    balances[msg.sender] -= amount;
+}
+```
+
+### Your Tasks:
+1. Write an attack contract that exploits this vulnerability
+2. Demonstrate the exploit using a test
+3. Fix the contract using:
+    - Checks-Effects-Interactions pattern
+    - Reentrancy guard
+4. Write tests proving:
+    - Attack fails after fix
+    - Legit users can still withdraw
\ No newline at end of file
diff --git a/13-04-26-test/script/Counter.s.sol b/13-04-26-test/script/Counter.s.sol
new file mode 100644
index 00000000..f01d69c3
--- /dev/null
+++ b/13-04-26-test/script/Counter.s.sol
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.13;
+
+import {Script} from "forge-std/Script.sol";
+import {Counter} from "../src/Counter.sol";
+
+contract CounterScript is Script {
+    Counter public counter;
+
+    function setUp() public {}
+
+    function run() public {
+        vm.startBroadcast();
+
+        counter = new Counter();
+
+        vm.stopBroadcast();
+    }
+}
diff --git a/13-04-26-test/src/Attacker.sol b/13-04-26-test/src/Attacker.sol
new file mode 100644
index 00000000..34f0179b
--- /dev/null
+++ b/13-04-26-test/src/Attacker.sol
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.28;
+
+import {VulnerableContract} from "./VulnerableContract.sol";
+
+contract Attacker {
+    VulnerableContract public vulnerableContract;
+
+    constructor(address _vcAddress) {
+        vulnerableContract = VulnerableContract(_vcAddress);
+    }
+
+    function attack() external payable {
+        require(msg.value > 0, "Need ETH to attack");
+        vulnerableContract.deposit{value: msg.value}();
+        vulnerableContract.withdraw(msg.value);
+    }
+
+    receive() external payable {
+        if (address(vulnerableContract).balance >= msg.value) {
+            vulnerableContract.withdraw(msg.value);
+        }
+    }
+}
\ No newline at end of file
diff --git a/13-04-26-test/src/FixedContract.sol b/13-04-26-test/src/FixedContract.sol
new file mode 100644
index 00000000..e8cb3077
--- /dev/null
+++ b/13-04-26-test/src/FixedContract.sol
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.28;
+
+contract FixedContract {
+    mapping(address => uint256) public balances;
+
+    bool private locked;
+
+    modifier noReentrant() {
+        require(!locked, "No re-entrancy");
+        locked = true;
+        _;
+        locked = false;
+    }
+
+    function deposit() public payable {
+        balances[msg.sender] += msg.value;
+    }
+
+    function withdraw(uint256 amount) public {
+        // checks
+        require(balances[msg.sender] >= amount, "Insufficient balance");
+
+        // effects
+        balances[msg.sender] -= amount;
+
+        // interactions
+        (bool success, ) = msg.sender.call{value: amount}("");
+        require(success, "Transfer failed");
+    }
+}
\ No newline at end of file
diff --git a/13-04-26-test/src/VulnerableContract.sol b/13-04-26-test/src/VulnerableContract.sol
new file mode 100644
index 00000000..88739381
--- /dev/null
+++ b/13-04-26-test/src/VulnerableContract.sol
@@ -0,0 +1,19 @@
+//SPDX-License-Identifier: MIT
+pragma solidity ^0.8.28;
+
+contract VulnerableContract {
+    mapping(address => uint256) public balances;
+
+    function deposit() public payable {
+        balances[msg.sender] += msg.value;
+    }
+
+    function withdraw(uint256 amount) public {
+        require(balances[msg.sender] >= amount, "Insufficient balance");
+
+        (bool success, ) = msg.sender.call{value: amount}("");
+        require(success, "Transfer failed");
+
+        balances[msg.sender] -= amount;
+    }
+}
\ No newline at end of file
diff --git a/13-04-26-test/test/VulnerableContract.t.sol b/13-04-26-test/test/VulnerableContract.t.sol
new file mode 100644
index 00000000..1714c1be
--- /dev/null
+++ b/13-04-26-test/test/VulnerableContract.t.sol
@@ -0,0 +1,91 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.28;
+
+import {Test, console} from "forge-std/Test.sol";
+import {VulnerableContract} from "../src/VulnerableContract.sol";
+import {FixedContract} from "../src/FixedContract.sol";
+import {Attacker} from "../src/Attacker.sol";
+
+contract VulnerableContractTest is Test {
+    VulnerableContract public vulnerable;
+    FixedContract public fixedContract; 
+    Attacker public attacker;
+
+    function setUp() public {
+        vulnerable = new VulnerableContract();
+        fixedContract = new FixedContract();
+        attacker = new Attacker(address(vulnerable));
+    }
+
+    function test_ExploitDrainsVulnerable() public {
+        // 1. Give a random user 10 ETH and have them deposit it
+        address user = address(0x123);
+        vm.deal(user, 10 ether);
+        vm.prank(user);
+        vulnerable.deposit{value: 10 ether}();
+
+        // 2. Give the Attacker contract 1 ETH to start the attack
+        vm.deal(address(attacker), 1 ether);
+        vm.prank(address(attacker));
+        attacker.attack{value: 1 ether}();
+
+        // 3. Check if the bank is now empty
+        assertEq(address(vulnerable).balance, 0);
+    }
+
+    function test_LegitUserCanWithdrawFromFixed() public {
+        address legitUser = address(0x456);
+        
+        // Give user money and deposit
+        vm.deal(legitUser, 5 ether);
+        vm.prank(legitUser);
+        fixedContract.deposit{value: 5 ether}();
+
+        // Withdraw money
+        vm.prank(legitUser);
+        fixedContract.withdraw(5 ether);
+
+        // Check if balance is 0
+        assertEq(address(fixedContract).balance, 0);
+    }
+
+    function test_AttackFailsOnFixed() public {
+        // 1. Put money in the fixed bank
+        address user = address(0x1);
+        vm.deal(user, 10 ether);
+        vm.prank(user);
+        fixedContract.deposit{value: 10 ether}();
+
+        // 2. Deploy a mini attacker for the fixed bank
+        FixedAttacker fixedAttacker = new FixedAttacker(fixedContract);
+
+        // 3. Give it 1 ETH and tell it to attack
+        vm.deal(address(fixedAttacker), 1 ether);
+        vm.prank(address(fixedAttacker));
+        
+        // 4. Tell Foundry we expect this to fail
+        vm.expectRevert();
+        fixedAttacker.attack{value: 1 ether}();
+
+        // 5. Verify the bank still has its 10 ETH
+        assertEq(address(fixedContract).balance, 10 ether);
+    }
+
+    receive() external payable {}
+}
+
+contract FixedAttacker {
+    FixedContract public bank;
+    constructor(FixedContract _bank) { bank = _bank; }
+    
+    function attack() external payable {
+        bank.deposit{value: msg.value}();
+        bank.withdraw(msg.value);
+    }
+    
+    receive() external payable {
+        if (address(bank).balance >= msg.value) {
+            bank.withdraw(msg.value);
+        }
+    }
+}
\ No newline at end of file