How Angular Preventing Cross Site Scripting (XSS) and CSRF Attacks?

How Angular prevents this Attacks?

The Cross Site Scripting (XSS) attack is a type of injection and attackers inject your web applications using the client side scripts and malicious code into web pages.

An attacker can insert vulnerability scripts and malicious code in your web applications.

The Angular treats all values as un-trusted by default. This is the great advantages of Angular.

Stayed Informed – Angular 4 docs and Angular 5 docs

When a value is Inserted Vulnerability into the DOM from –

1. A Template

2. Property

3. Attribute

4. Style

5. Class Binding

6. Interpolation

7. And so on.

Angular recognizes the value as unsafe and automatically sanitizes and removes the script tag and other security vulnerabilities.

Angular provides built-in, values as untrusted by default, anti XSS and CSRF/XSRF protection.

The CookieXSRFStrategy class takes care of preventing XSS and CSRF/XSRF attacks.

The DomSanitizationService takes care of removing the dangerous bits in order to prevent XSS attacks.

Angular applications must follow the same security principles as regular web applications -

1. You should avoid direct use of the DOM APIs.

2. You should enable Content Security Policy (CSP) and configure your web server to return appropriate CSP HTTP headers.

3. You should Use the offline template compiler.

4. You should Use Server Side XSS protection.

5. You should Use DOM Sanitizer.

6. You should Preventing CSRF or XSRF attacks.

Angular defines the following security -

HTML is used when interpreting a value as HTML i.e.

Style is used when binding CSS into the style property i.e.

URL is used for URL properties i.e.

Resource URL is a URL that will be loaded and executed i.e.

<p class="e2e-inner-html-interpolated">{{htmlSnippet}}</p>

Example –

export const BROWSER_SANITIZATION_PROVIDERS: Array<any> = [

{provide: Sanitizer, useExisting: DomSanitizer},

{provide: DomSanitizer, useClass: DomSanitizerImpl},

];

@NgModule({

providers: [

BROWSER_SANITIZATION_PROVIDERS

...

exports: [CommonModule, ApplicationModule]

})

export class BrowserModule {}

DOM sanitization - Use to clean untrusted parts of values -

export enum SecurityContext { NONE, HTML, STYLE, SCRIPT, URL, RESOURCE_URL }

export abstract class DomSanitizer implements Sanitizer {

abstract sanitize(context: SecurityContext, value: SafeValue|string|null): string|null;

abstract bypassSecurityTrustHtml(value: string): SafeHtml;

abstract bypassSecurityTrustStyle(value: string): SafeStyle;

abstract bypassSecurityTrustScript(value: string): SafeScript;

abstract bypassSecurityTrustUrl(value: string): SafeUrl;

abstract bypassSecurityTrustResourceUrl(value: string): SafeResourceUrl;

}

The DOM Sanitize Methods –

sanitize(ctx: SecurityContext, value: SafeValue|string|null): string|null {

if (value == null) return null;

switch (ctx) {

case SecurityContext.NONE:

return value as string;

case SecurityContext.HTML:

if (value instanceof SafeHtmlImpl) return value.changingThisBreaksApplicationSecurity;

this.checkNotSafeValue(value, 'HTML');

return sanitizeHtml(this._doc, String(value));

case SecurityContext.STYLE:

if (value instanceof SafeStyleImpl) return value.changingThisBreaksApplicationSecurity;

this.checkNotSafeValue(value, 'Style');

return sanitizeStyle(value as string);

case SecurityContext.SCRIPT:

if (value instanceof SafeScriptImpl) return value.changingThisBreaksApplicationSecurity;

this.checkNotSafeValue(value, 'Script');

throw new Error('unsafe value used in a script context');

case SecurityContext.URL:

if (value instanceof SafeResourceUrlImpl || value instanceof SafeUrlImpl) {

// Allow resource URLs in URL contexts, they are strictly more trusted.

return value.changingThisBreaksApplicationSecurity;

}

this.checkNotSafeValue(value, 'URL');

return sanitizeUrl(String(value));

case SecurityContext.RESOURCE_URL:

if (value instanceof SafeResourceUrlImpl) {

return value.changingThisBreaksApplicationSecurity;

}

this.checkNotSafeValue(value, 'ResourceURL');

throw new Error(

'unsafe value used in a resource URL context (see http://g.co/ng/security#xss)');

default:

throw new Error(`Unexpected SecurityContext ${ctx} (see http://g.co/ng/security#xss)`);

}

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