diff --git a/Writeup/doublespending/day23/Ans.sol b/Writeup/doublespending/day23/Ans.sol
new file mode 100644
index 00000000..29593847
--- /dev/null
+++ b/Writeup/doublespending/day23/Ans.sol
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "./WBC.sol";
+import {console} from "forge-std/Test.sol";
+
+contract Ans {
+    WBC public immutable wbc;
+
+    constructor(address wbc_) {
+        wbc = WBC(wbc_);
+        wbc.bodyCheck();
+    }
+
+    function win() external {
+        wbc.ready();
+    }
+
+    function judge() external view returns (address) {
+        return block.coinbase;
+    }
+
+    function steal() external pure returns (uint160) {
+        return 507778882907781185490817896798523593512684789769;
+    }
+
+    function execute() external pure returns (bytes32) {
+        string memory ans = "HitAndRun";
+        return bytes32(uint256(uint80(bytes10(abi.encodePacked(uint8(bytes(ans).length), ans)))));
+    }
+
+    function shout() external view returns (string memory) {
+        if (gasleft() % 3 == 2) {
+            return "I'm the best";
+        } else {
+            return "We are the champion!";
+        }
+    }
+}
diff --git a/Writeup/doublespending/day23/WBC.t.sol b/Writeup/doublespending/day23/WBC.t.sol
new file mode 100644
index 00000000..d3743965
--- /dev/null
+++ b/Writeup/doublespending/day23/WBC.t.sol
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import {console2} from "forge-std/console2.sol";
+import {Test} from "forge-std/Test.sol";
+import {WBC, WBCBase} from "src/WBC/WBC.sol";
+import {Ans} from "src/WBC/Ans.sol";
+
+contract WBCTest is Test {
+    WBCBase public base;
+    WBC public wbc;
+    Ans public ans;
+
+    uint256 count;
+
+    function setUp() external {
+        uint256 startTime = block.timestamp + 60;
+        uint256 endTime = startTime + 60;
+        uint256 fullScore = 100;
+        base = new WBCBase(startTime, endTime, fullScore);
+        base.setup();
+        wbc = base.wbc();
+    }
+
+    function testExploit() external {
+        for (uint256 i = 0; i < 1000; ++i) {
+            try new Ans{salt: bytes32(i)}(address(wbc)) returns (Ans _ans) {
+                ans = _ans;
+                break;
+            } catch {}
+        }
+        ans.win();
+        base.solve();
+        assertTrue(base.isSolved());
+    }
+
+    function testCannotHomeRunEasily() external {
+        vm.expectRevert("try again");
+        wbc.homerun();
+        vm.expectRevert();
+        base.solve();
+    }
+}
diff --git a/doublespending.md b/doublespending.md
index 67cdaa85..6013028c 100644
--- a/doublespending.md
+++ b/doublespending.md
@@ -325,4 +325,35 @@ B: [EthTaipei CTF 2023](https://github.com/dinngo/ETHTaipei-war-room/)(5)
   - Then, [solidity (version < 0.8.0)](https://github.com/consenlabs/account-abstraction/blob/3381bd75823c238a05c870fb11c61548c8fc7c9d/src/paymaster/OffChainPaymaster.sol#L2) has not applied underflow check as default.
   - Finally, we can withdraw twice using `onERC721Received` callback and make the balance underflow.
 
+### 2024.09.20
+
+B: [EthTaipei CTF 2023](https://github.com/dinngo/ETHTaipei-war-room/)(5)
+
+- WBC
+  - We should create contract `Ans` implmenting `IGame` to satisfy the requirements of `WBC`
+  - To pass `bodyCheck`
+    - Call `bodyCheck` inside `constructor` of `Ans` to meet the requirement [here](https://github.com/dinngo/ETHTaipei-war-room/blob/b5bdb72097172f50baa13b996be2422fd1b6786c/src/WBC/WBC.sol#L32)
+    - Use differnt salt to create `Ans` with different addresses to meet the requirement [here](https://github.com/dinngo/ETHTaipei-war-room/blob/b5bdb72097172f50baa13b996be2422fd1b6786c/src/WBC/WBC.sol#L33)
+  - To pass `ready`
+    - `judge()` of `Ans` should return `block.conbase`.
+  - To pass `_swing`
+    - To pass `_secondBase`
+    - `steal()` of `Ans` should return the `input` value by just copy
+    - To pass `_thirdBase` - We should find xxx that `this.decode(xxx) = "HitAndRun"`.
+      - xxx = 0\*(32-10)||9||HitAndRun
+        ```
+          function decode(bytes32 data) external pure returns (string memory) {
+            assembly {
+                mstore(0x20, 0x20)
+                mstore(0x49, data)
+                // [0x20(offset), 0*9 || data[0:23](length), data[23:32] || 0*23(raw)]
+                // [0x20(offset), 0*9 || xxx.length(length), xxx.data || 0*23(raw)]
+                return(0x20, 0x60)
+            }
+          }
+        ```
+    - To pass `_homeBase`
+      - We should find a way to distinguish the two sequential static call. - We can use `gasleft()`
+      - We can find a value `i` - `gasleft() % i == 0` in the first call - `gasleft() % i != 0` in the second call
+
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