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Sanitizing User Input

For components such as ion-alert developers can allow for custom or user-provided content. This content can be plain text or HTML and should be considered untrusted. As with any untrusted input, it is important to sanitize it before doing anything else with it. In particular, using things like innerHTML without sanitization provides an attack vector for bad actors to input malicious content and potentially launch a Cross Site Scripting attack (XSS).

Ionic comes built in with basic sanitization methods for the components it provides, but for user-created components it is up to the developer to make sure all data is sanitized. Different frameworks have different solutions for sanitizing user input, so developers should familiarize themselves with what their specific framework offers.

For developers who are not using a framework, or for developers whose framework does not provide the sanitization methods they need, we recommend using sanitize-html. This package provides a simple HTML sanitizer that allows the developer to specify the exact tags and attributes that they want to allow in their application.

Angular

Angular comes built in with the DomSanitizer class. This helps prevent XSS issues by ensuring that values are safe to be used in the DOM. By default, Angular will mark any values it deems unsafe. For example, the following link would be marked as unsafe by Angular because it would attempt to execute some JavaScript.

public myUrl: string = 'javascript:alert("oh no!")';

...

<a [href]="myUrl">Click Me!</a>

To learn more about the built-in protections that Angular provides, see the Angular Security Guide.

React

React DOM escapes values embedded in JSX before rendering them by converting them to strings. For example, the following would be safe as name is converted to a string before being rendered:

const name = values.name;
const element = <h1>Hello, {name}!</h1>;

However, this does not stop someone from injecting JavaScript into places such as the href attribute of an anchor element. The following is unsafe and can potentially allow an XSS attack to occur:

const userInput = 'javascript:alert("Oh no!")';
const element = <a href={userInput}>Click Me!</a>

If the developer needs to achieve more comprehensive sanitization, they can use the sanitize-html package.

To learn more about the built-in protections that React and JSX provide, see the React JSX Documentation.

Vue

Vue does not provide any type of sanitizing methods built in. It is recommended that developers use a package such as sanitize-html.

To learn more about the security recommendations for binding to directives such as v-html, see the Vue Syntax Guide.

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