> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/web-exploitation/some-assembly-required1.md).
# some-assembly-required1
### 🔍 Challenge Overview
We are given a web application that validates a flag using client-side logic. The JavaScript file is heavily obfuscated and loads a **WebAssembly (.wasm)** binary for validation.
### 🧩 Step 1: Analyze JavaScript
We fetch the JS file:
```bash
curl -v http://wily-courier.picoctf.net:53140/G82XCw5CX3.js
```
#### Key Observations:
* The code is **obfuscated**, but we can still identify:
* `fetch("./JIFxzHyW8W")` → loads a WASM file
* `WebAssembly.instantiate(...)`
* Functions used:
* `copy_char`
* `check_flag`
#### Logic:
```javascript
for (let i = 0; i < input.length; i++) {
exports.copy_char(input.charCodeAt(i), i);
}
exports.copy_char(0, input.length);
if (exports.check_flag() == 1) {
"Correct!"
} else {
"Incorrect!"
}
```
👉 So:
* Input is passed character-by-character into WASM memory
* `check_flag()` validates it
***
### 🧩 Step 2: Extract WASM File
```bash
curl -O http://wily-courier.picoctf.net:53140/JIFxzHyW8W
```
Check file type:
```bash
file JIFxzHyW8W
```
Output:
```
WebAssembly (wasm) binary module
```
***
### 🧩 Step 3: Reverse / Inspect WASM
Instead of fully reversing (hard), we use a shortcut:
```bash
strings JIFxzHyW8W
```
#### Output:
```
picoCTF{68ff4bc93f6d67637b2f471be209d132}
```
🔥 The flag is directly embedded in the binary.
***
### 🏁 Final Flag
```
picoCTF{68ff4bc93f6d67637b2f471be209d132}
```
***
### 💡 Key Takeaways
* **WebAssembly ≠ secure** → client-side validation can always be reversed
* Always check:
* JS files
* WASM binaries
* Network requests
* `strings` is OP for beginner CTFs
* If logic looks complex → **look for shortcuts first**
---
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