block includes
  include ../_util-fns
:marked
  This section describes Angular's built-in
  protections against common web application vulnerabilities and attacks such as cross-site
  scripting attacks. It does not cover application-level security, such as authentication (_Who is
  this user?_) or authorization (_What can this user do?_).

  For more information about the attacks and mitigations described below, see [OWASP Guide Project](https://www.owasp.org/index.php/Category:OWASP_Guide_Project).

.l-main-section
:marked
  # Contents:

  * [Reporting vulnerabilities](#report-issues).
  * [Best practices](#best-practices).
  * [Preventing cross-site scripting (XSS)](#xss).
  * [Trusting safe values](#bypass-security-apis).
  * [HTTP-Level vulnerabilities](#http).
  * [Auditing Angular applications](#code-review).

  Try the <live-example></live-example> of the code shown in this page.

.l-main-section
h2#report-issues Reporting vulnerabilities
:marked
  Email us at [security@angular.io](mailto:security@angular.io) to report vulnerabilities in
  Angular itself.

  For more information about how Google handles security issues, see [Google's security
  philosophy](https://www.google.com/about/appsecurity/).

.l-main-section
h2#best-practices Best practices
:marked
  * **Keep current with the latest Angular library releases.**
  We regularly update our Angular libraries, and these updates may fix security defects discovered in
  previous versions. Check the Angular [change
  log](https://github.com/angular/angular/blob/master/CHANGELOG.md) for security-related updates.

  * **Don't modify your copy of Angular.**
  Private, customized versions of Angular tend to fall behind the current version and may not include
  important security fixes and enhancements. Instead, share your Angular improvements with the
  community and make a pull request.

  * **Avoid Angular APIs marked in the documentation as “[_Security Risk_](#bypass-security-apis).”**

.l-main-section
h2#xss Preventing cross-site scripting (XSS)
:marked
  [Cross-site scripting (XSS)](https://en.wikipedia.org/wiki/Cross-site_scripting) enables attackers
  to inject malicious code into web pages. Such code can then, for example, steal user data (in
  particular, their login data) or perform actions impersonating the user. This is one of the most
  common attacks on the web.

  To block XSS attacks, you must prevent malicious code from entering the DOM (Document Object Model). For example, if an
  attacker can trick you into inserting a `<script>` tag in the DOM, they can run arbitrary code on
  your website. The attack is not limited to `<script>` tags&mdash;many elements and properties in the
  DOM allow code execution, for example, `<img onerror="...">` and `<a href="javascript:...">`. If
  attacker-controlled data enters the DOM, expect security vulnerabilities.

  ### Angular’s cross-site scripting security model

  To systematically block XSS bugs, Angular treats all values as untrusted by default. When a value
  is inserted into the DOM from a template, via property, attribute, style, class binding, or interpolation, Angular sanitizes and escapes untrusted values.

  _Angular templates are the same as executable code_: HTML, attributes, and binding expressions
  (but not the values bound!) in templates are trusted to be safe. This means that applications must
  prevent values that an attacker can control from ever making it into the source code of a
  template. Never generate template source code by concatenating user input and templates! Using
  the [offline template compiler](#offline-template-compiler) is an effective way to prevent these
  vulnerabilities, also known as _template injection_.

  ### Sanitization and security contexts

  _Sanitization_ is the inspection of an untrusted value, turning it into a value that is safe to insert into
  the DOM. In many cases, sanitization does not change a value at all. Sanitization depends on context:
  a value that is harmless in CSS is potentially dangerous in a URL.

  Angular defines four security contexts&mdash;HTML, style, URL, and resource URL:

  * **HTML** is used when interpreting a value as HTML, for example, when binding to `innerHtml`
  * **Style** is used when binding CSS into the `style` property
  * **URL** is used for URL properties such as `<a href>`
  * **Resource URL** is a URL that will be loaded and executed as code, for example, in `<script src>`

  Angular sanitizes untrusted values for the first three items; sanitizing resource URLs is not
  possible because they contain arbitrary code. In development mode, Angular prints a console warning
  when it has to change a value during sanitization.

  ### Sanitization example

  The template below binds the value of `htmlSnippet`, once by interpolating it into an element's
  content, and once by binding it to the `innerHTML` property of an element:

+makeExample('app/inner-html-binding.component.html')

:marked
  Interpolated content is always escaped&mdash;the HTML is not interpreted, and the browser displays
  angle brackets in the element's text content.

  For the HTML to be interpreted, you must bind it to an HTML property such as `innerHTML`. But binding
  a value that an attacker might control into `innerHTML` normally causes an XSS
  vulnerability. For example, code contained in a `<script>` tag is executed:

+makeExcerpt('app/inner-html-binding.component.ts ()', 'inner-html-controller')

:marked
  Angular recognizes the value as unsafe and automatically sanitizes it, which removes the `<script>`
  tag but keeps safe content such as the text content of the `<script>` tag, or the `<b>` element.

figure.image-display
    img(src='/resources/images/devguide/security/binding-inner-html.png'
        alt='A screenshot showing interpolated and bound HTML values')
:marked
  ### Avoid direct use of the DOM APIs

  The built-in browser DOM APIs do not automatically protect you from security vulnerabilities.
  For example, `document`, the node available through `ElementRef`, and many third-party APIs
  contain unsafe methods. Avoid directly interacting with the DOM and instead use Angular
  templates where possible.

  ### Content security policy

  [Content Security Policy (CSP)](http://www.html5rocks.com/en/tutorials/security/content-security-policy/) is a defense-in-depth
  technique to prevent XSS. To enable CSP, configure your web server to return an appropriate
  `Content-Security-Policy` HTTP header.

  <a id="offline-template-compiler"></a>
  ### Use the offline template compiler

  The offline template compiler prevents a whole class of vulnerabilities called template injection,
  and also greatly improves application performance. Use the offline template compiler in production
  deployments; do not dynamically generate templates. Angular trusts template code, so generating
  templates, in particular templates containing user data, circumvents Angular's built-in protections. For information about how to dynamically construct forms in a safe way, see 
  [Dynamic Forms Cookbook](../cookbook/dynamic-form.html).

  ### Server-side XSS protection

  HTML constructed on the server is vulnerable to injection attacks. Injecting template code into an
  Angular application is the same as injecting executable code into the
  application: it gives the attacker full control over the application. To prevent this, 
  use a templating language that automatically escapes values to prevent XSS vulnerabilities on
  the server. Do not generate Angular templates on the server side using a templating language; doing this
  carries a high risk of introducing template-injection vulnerabilities.

.l-main-section
h2#bypass-security-apis Trusting safe values
:marked
  Sometimes applications genuinely need to include executable code, display an `<iframe>` from some
  URL, or construct potentially dangerous URLs. To prevent automatic sanitization in any of these 
  situations, you can tell Angular that you inspected a value, checked how it was generated, and made 
  sure it will always be secure. But **be careful**! If you trust a value that might be malicious, you 
  are introducing a security vulnerability into your application. If in doubt, find a professional 
  security reviewer.

  You can mark a value as trusted by injecting `DomSanitizer` and calling one of the
  following methods:

  * `bypassSecurityTrustHtml`
  * `bypassSecurityTrustScript`
  * `bypassSecurityTrustStyle`
  * `bypassSecurityTrustUrl`
  * `bypassSecurityTrustResourceUrl`

  Remember, whether a value is safe depends on context, so you need to choose the right context for
  your intended use of the value. Imagine that the following template needs to bind a URL to a
  `javascript:alert(...)` call:

+makeExcerpt('app/bypass-security.component.html ()', 'dangerous-url')

:marked
  Normally, Angular automatically sanitizes the URL, disables the dangerous code, and
  in development mode, logs this action to the console. To prevent
  this, you can mark the URL value as a trusted URL using the `bypassSecurityTrustUrl` call:

+makeExcerpt('app/bypass-security.component.ts ()', 'trust-url')

figure.image-display
    img(src='/resources/images/devguide/security/bypass-security-component.png'
        alt='A screenshot showing an alert box created from a trusted URL')

:marked
  If you need to convert user input into a trusted value, use a
  controller method. The template below allows users to enter a YouTube video ID and load the
  corresponding video in an `<iframe>`. The `<iframe src>` attribute is a resource URL security
  context, because an untrusted source can, for example, smuggle in file downloads that unsuspecting users 
  could execute. So call a method on the controller to construct a trusted video URL, that causes
  Angular to then allow binding into `<iframe src>`:

+makeExcerpt('app/bypass-security.component.html ()', 'iframe-videoid')
+makeExcerpt('app/bypass-security.component.ts ()', 'trust-video-url')

.l-main-section
h2#http HTTP-level vulnerabilities
:marked
  Angular has built-in support to help prevent two common HTTP vulnerabilities, cross-site request
  forgery (CSRF or XSRF) and cross-site script inclusion (XSSI). Both of these must be mitigated primarily 
  on the server side, but Angular ships helpers to make integration on the client side easier.

h3#xsrf Cross-site request forgery
:marked
  In a cross-site request forgery, an attacker tricks the user into visiting a
  _different_ page and has them, for example, submit a form that sends a request to your application's
  web server. If the user is logged into your application, the browser will send authentication
  cookies, and the attacker could&mdash;for example&mdash;cause a bank transfer in the user's name with
  the right request.

  To prevent this, your application must ensure that user requests originate in your own
  application, not on a different site. A common technique is that the server sends a randomly
  generated authentication token in a cookie, often with the name `XSRF-TOKEN`. Only the website 
  on which cookies are set can read the cookies, so only your own application can read this token. On
  each API request, the server then validates the client by checking that the token is sent back,
  usually in an HTTP header called `X-XSRF-TOKEN`.

  The Angular `http` client has built-in support for this technique. The default
  `CookieXSRFStrategy` looks for a cookie called `XSRF-TOKEN` and sets an HTTP request header named
  `X-XSRF-TOKEN` with the value of that cookie on every request. The server must set the
  `XSRF-TOKEN` cookie and validate the response header for each state-modifying request.

  CSRF tokens should be unique per user and session, have a large random value generated by a
  cryptographically secure random number generator, and expire.

  Angular applications can customize cookie and header names by binding their own
  `CookieXSRFStrategy` value or implement an entirely custom `XSRFStrategy` through providing a custom
  binding for that type by adding either of the following to your providers list:

code-example(language="typescript").
  { provide: XSRFStrategy, useValue: new CookieXSRFStrategy('myCookieName', 'My-Header-Name')}
  { provide: XSRFStrategy, useClass: MyXSRFStrategy}

:marked
  For information about CSRF at the Open Web Application Security Project (OWASP) see
  [Cross-Site Request Forgery (CSRF)](https://www.owasp.org/index.php/Cross-Site_Request_Forgery_%28CSRF%29) and
  [Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet](https://www.owasp.org/index.php/CSRF_Prevention_Cheat_Sheet). The Stanford University
  paper [Robust Defenses for Cross-Site Request Forgery](https://seclab.stanford.edu/websec/csrf/csrf.pdf) is also a rich source of detail.

h3#xssi Cross-site script inclusion (XSSI)
:marked
  Cross-site script inclusion, also known as JSON vulnerability, can allow an attacker's website to
  read data from a JSON API. The attack works on older browsers by overriding native JavaScript
  object constructors, and then including an API URL using a `<script>` tag.

  This attack is only successful if the returned JSON is executable as JavaScript. Servers can
  prevent an attack by prefixing all JSON responses to make them non-executable, by convention, using the
  well-known string `")]}',\n"`.

  Angular's `Http` library recognizes this convention and automatically strips the string
  `")]}',\n"` from all responses before further parsing.

  For more information, see the XSSI section of this [Google web security blog
  post](https://security.googleblog.com/2011/05/website-security-for-webmasters.html).

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h2#code-review Auditing angular applications
:marked
  Angular applications must follow the same security principles as regular web applications, and
  must be audited as such. Angular-specific APIs that should be audited in a security review,
  such as the [_bypassSecurityTrust_](#bypass-security-apis) methods, are marked in the documentation
  as security sensitive.