It is like a [**Server Side Template Injection**](ssti-server-side-template-injection/) but in the **client**. The **SSTI** can allow you the **execute code** on the remote server, the **CSTI** could allow you to **execute arbitrary JavaScript** code in the victim.
The way to **test** for this vulnerability is very **similar** as in the case of **SSTI**, the interpreter is going to expect something to execute **between doubles keys** and will execute it. For example using something like: `{{ 7-7 }}` if the server is **vulnerable** you will see a `0` and if not you will see the original: `{{ 7-7 }}`
### AngularJS
AngularJS is a popular JavaScript library, which scans the contents of HTML nodes containing the **`ng-app`** attribute \(also known as an AngularJS directive\). When a directive is added to the HTML code, **you can execute JavaScript expressions within double curly braces**.
For example, if your **input** is being **reflected** inside the **body** of the HTML and the body is defined with `ng-app`: **`<body ng-app>`**
You can **execute arbitrary JavaScript** code using curly braces **adding** to the **body**:
\*\*\*\*[**Angular 1.6 removed the sandbox**](http://blog.angularjs.org/2016/09/angular-16-expression-sandbox-removal.html#:~:text=The%20Angular%20expression%20sandbox%20will,smaller%20and%20easier%20to%20maintain.&text=Removing%20the%20expression%20sandbox%20does,surface%20of%20Angular%201%20applications.) so from this version a payload like `{{constructor.constructor('alert(1)')()}}` or `<input ng-focus=$event.view.alert('XSS')>` should work.
You can find a **vulnerable vue.js** implementation in [https://vue-client-side-template-injection-example.azu.now.sh/](https://vue-client-side-template-injection-example.azu.now.sh/)
Working payload: [`https://vue-client-side-template-injection-example.azu.now.sh/?name=%7B%7Bthis.constructor.constructor(%27alert(%22foo%22)%27)()%7D%`](https://vue-client-side-template-injection-example.azu.now.sh/?name=%7B%7Bthis.constructor.constructor%28%27alert%28%22foo%22%29%27%29%28%29%7D%7D)
And the **source code** of the vulnerable example here: [https://github.com/azu/vue-client-side-template-injection-example](https://github.com/azu/vue-client-side-template-injection-example)