3814. Prevent Security Vulnerabilities in Web Development
DomSanitizer and JSX

Develop web application in secure way when using angular, react, etc.

1. Website Security

  • XSS
  • DDOS
  • CSRF

2. Angular

In Angular, class DomSanitizer helps preventing Cross Site Scripting Security bugs (XSS) by sanitizing values to be safe to use in the different DOM contexts.

2.1 Bypass Security Check

Calling any of the bypassSecurityTrust... APIs disables Angular’s built-in sanitization for the value passed in. Carefully check and audit all values and code paths going into this call. Make sure any user data is appropriately escaped for this security context.

  • bypassSecurityTrustHtml
  • bypassSecurityTrustScript
  • bypassSecurityTrustStyle
  • bypassSecurityTrustUrl
  • bypassSecurityTrustResourceUrl


import {BrowserModule, DomSanitizer} from '@angular/platform-browser'

  selector: 'my-app',
  template: `
     <div [innerHtml]="html"></div>
export class App {
  constructor(private sanitizer: DomSanitizer) {
    this.html = sanitizer.bypassSecurityTrustHtml('<h1>DomSanitizer</h1><script>ourSafeCode()</script>') ;

2.2 Sanitize HTML

Calling sanitize method to manually sanitize html, css, url, etc.

  • SecurityContext.NONE
  • SecurityContext.HTML
  • SecurityContext.STYLE
  • SecurityContext.SCRIPT
  • SecurityContext.URL
  • SecurityContext.RESOURCE_URL


import {Component, SecurityContext} from '@angular/core'
export class App {
  constructor(private sanitizer: DomSanitizer) {
    this.html = sanitizer.sanitize(SecurityContext.HTML, "<h1>Sanitize</h1><script>attackerCode()</script>");

3. React

3.1 JSX Prevents Injection Attacks

By default, React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered. This helps prevent XSS (cross-site-scripting) attacks.

It is safe to embed user input in JSX:

const title = response.potentiallyMaliciousInput;
// This is safe:
const element = <h1>{title}</h1>;

Example for how JSX works
React automatically escapes variables for you. It prevents XSS injection via string HTML with malicious Javascript. Naturally, inputs are sanitized along with this.

For instance, let’s say you have this string.

var htmlString = '<img src="javascript:alert('XSS!')" />';

If you try to render this string in react,

render() {
    return (

you will literally see on the page the whole string including the element tag. aka in the browser you will see

<img src="javascript:alert('XSS!')" />

If you view the source html you would see

<span>"<img src="javascript:alert('XSS!')" />"</span>

3.2 Other XSS Attacks

But JSX can’t prevent all attacks, see below.
1) XSS via dangerouslySetInnerHTML

const aboutUserText = "<img onerror='alert(\"Hacked!\");' src='invalid-image' />";

class AboutUserComponent extends React.Component {
  render() {
    return (
      <div dangerouslySetInnerHTML={{"__html": aboutUserText}} />

ReactDOM.render(<AboutUserComponent />, document.querySelector("#app"))

2) XSS via a.href attribute

const userWebsite = "javascript:alert('Hacked!');";

class UserProfilePage extends React.Component {
  render() {
    return (
      <a href={userWebsite}>My Website</a>

ReactDOM.render(<UserProfilePage />, document.querySelector("#app"));

3) XSS via attacker controlled props

const customPropsControledByAttacker = {
  dangerouslySetInnerHTML: {
    "__html": "<img onerror='alert(\"Hacked!\");' src='invalid-image' />"

class Divider extends React.Component {
  render() {
    return (
      <div {...customPropsControledByAttacker} />

ReactDOM.render(<Divider />, document.querySelector("#app"));

4. References