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$sce

  1. - $sceProvider
  2. - service in module ng

$sce is a service that provides Strict Contextual Escaping services to AngularJS.

Strict Contextual Escaping

Strict Contextual Escaping (SCE) is a mode in which AngularJS requires bindings in certain contexts to result in a value that is marked as safe to use for that context. One example of such a context is binding arbitrary html controlled by the user via ng-bind-html. We refer to these contexts as privileged or SCE contexts.

As of version 1.2, Angular ships with SCE enabled by default.

Note: When enabled (the default), IE<11 in quirks mode is not supported. In this mode, IE<11 allow one to execute arbitrary javascript by the use of the expression() syntax. Refer to learn more about them. You can ensure your document is in standards mode and not quirks mode by adding <!doctype html> to the top of your HTML document.

SCE assists in writing code in way that (a) is secure by default and (b) makes auditing for security vulnerabilities such as XSS, clickjacking, etc. a lot easier.

Here's an example of a binding in a privileged context:

<input ng-model="userHtml" aria-label="User input">
<div ng-bind-html="userHtml"></div>

Notice that ng-bind-html is bound to userHtml controlled by the user. With SCE disabled, this application allows the user to render arbitrary HTML into the DIV. In a more realistic example, one may be rendering user comments, blog articles, etc. via bindings. (HTML is just one example of a context where rendering user controlled input creates security vulnerabilities.)

For the case of HTML, you might use a library, either on the client side, or on the server side, to sanitize unsafe HTML before binding to the value and rendering it in the document.

How would you ensure that every place that used these types of bindings was bound to a value that was sanitized by your library (or returned as safe for rendering by your server?) How can you ensure that you didn't accidentally delete the line that sanitized the value, or renamed some properties/fields and forgot to update the binding to the sanitized value?

To be secure by default, you want to ensure that any such bindings are disallowed unless you can determine that something explicitly says it's safe to use a value for binding in that context. You can then audit your code (a simple grep would do) to ensure that this is only done for those values that you can easily tell are safe - because they were received from your server, sanitized by your library, etc. You can organize your codebase to help with this - perhaps allowing only the files in a specific directory to do this. Ensuring that the internal API exposed by that code doesn't markup arbitrary values as safe then becomes a more manageable task.

In the case of AngularJS' SCE service, one uses $sce.trustAs (and shorthand methods such as $sce.trustAsHtml, etc.) to obtain values that will be accepted by SCE / privileged contexts.

How does it work?

In privileged contexts, directives and code will bind to the result of $sce.getTrusted(context, value) rather than to the value directly. Directives use $sce.parseAs rather than $parse to watch attribute bindings, which performs the $sce.getTrusted behind the scenes on non-constant literals.

As an example, ngBindHtml uses $sce.parseAsHtml(binding expression). Here's the actual code (slightly simplified):

var ngBindHtmlDirective = ['$sce', function($sce) {
  return function(scope, element, attr) {
    scope.$watch($sce.parseAsHtml(attr.ngBindHtml), function(value) {
      element.html(value || '');
    });
  };
}];

Impact on loading templates

This applies both to the ng-include directive as well as templateUrl's specified by directives.

By default, Angular only loads templates from the same domain and protocol as the application document. This is done by calling $sce.getTrustedResourceUrl on the template URL. To load templates from other domains and/or protocols, you may either whitelist them or wrap it into a trusted value.

Please note: The browser's Same Origin Policy and Cross-Origin Resource Sharing (CORS) policy apply in addition to this and may further restrict whether the template is successfully loaded. This means that without the right CORS policy, loading templates from a different domain won't work on all browsers. Also, loading templates from file:// URL does not work on some browsers.

This feels like too much overhead

It's important to remember that SCE only applies to interpolation expressions.

If your expressions are constant literals, they're automatically trusted and you don't need to call $sce.trustAs on them (remember to include the ngSanitize module) (e.g. <div ng-bind-html="'<b>implicitly trusted</b>'"></div>) just works.

Additionally, a[href] and img[src] automatically sanitize their URLs and do not pass them through $sce.getTrusted. SCE doesn't play a role here.

The included $sceDelegate comes with sane defaults to allow you to load templates in ng-include from your application's domain without having to even know about SCE. It blocks loading templates from other domains or loading templates over http from an https served document. You can change these by setting your own custom whitelists and blacklists for matching such URLs.

This significantly reduces the overhead. It is far easier to pay the small overhead and have an application that's secure and can be audited to verify that with much more ease than bolting security onto an application later.

What trusted context types are supported?

Context Notes
$sce.HTML For HTML that's safe to source into the application. The ngBindHtml directive uses this context for bindings. If an unsafe value is encountered and the $sanitize module is present this will sanitize the value instead of throwing an error.
$sce.CSS For CSS that's safe to source into the application. Currently unused. Feel free to use it in your own directives.
$sce.URL For URLs that are safe to follow as links. Currently unused (<a href= and <img src= sanitize their urls and don't constitute an SCE context.
$sce.RESOURCE_URL For URLs that are not only safe to follow as links, but whose contents are also safe to include in your application. Examples include ng-include, src / ngSrc bindings for tags other than IMG (e.g. IFRAME, OBJECT, etc.)

Note that $sce.RESOURCE_URL makes a stronger statement about the URL than $sce.URL does and therefore contexts requiring values trusted for $sce.RESOURCE_URL can be used anywhere that values trusted for $sce.URL are required.
$sce.JS For JavaScript that is safe to execute in your application's context. Currently unused. Feel free to use it in your own directives.

Format of items in resourceUrlWhitelist/Blacklist

Each element in these arrays must be one of the following:

Refer $sceDelegateProvider for an example.

Show me an example using SCE.

  Edit in Plunker 
<div ng-controller="AppController as myCtrl">
  <i ng-bind-html="myCtrl.explicitlyTrustedHtml" id="explicitlyTrustedHtml"></i><br><br>
  <b>User comments</b><br>
  By default, HTML that isn't explicitly trusted (e.g. Alice's comment) is sanitized when
  $sanitize is available.  If $sanitize isn't available, this results in an error instead of an
  exploit.
  <div class="well">
    <div ng-repeat="userComment in myCtrl.userComments">
      <b>{{userComment.name}}</b>:
      <span ng-bind-html="userComment.htmlComment" class="htmlComment"></span>
      <br>
    </div>
  </div>
</div>
angular.module('mySceApp', ['ngSanitize'])
.controller('AppController', ['$http', '$templateCache', '$sce',
  function($http, $templateCache, $sce) {
    var self = this;
    $http.get("test_data.json", {cache: $templateCache}).success(function(userComments) {
      self.userComments = userComments;
    });
    self.explicitlyTrustedHtml = $sce.trustAsHtml(
        '<span onmouseover="this.textContent=&quot;Explicitly trusted HTML bypasses ' +
        'sanitization.&quot;">Hover over this text.</span>');
  }]);
[
  { "name": "Alice",
    "htmlComment":
        "<span onmouseover='this.textContent=\"PWN3D!\"'>Is <i>anyone</i> reading this?</span>"
  },
  { "name": "Bob",
    "htmlComment": "<i>Yes!</i>  Am I the only other one?"
  }
]
describe('SCE doc demo', function() {
  it('should sanitize untrusted values', function() {
    expect(element.all(by.css('.htmlComment')).first().getInnerHtml())
        .toBe('<span>Is <i>anyone</i> reading this?</span>');
  });

  it('should NOT sanitize explicitly trusted values', function() {
    expect(element(by.id('explicitlyTrustedHtml')).getInnerHtml()).toBe(
        '<span onmouseover="this.textContent=&quot;Explicitly trusted HTML bypasses ' +
        'sanitization.&quot;">Hover over this text.</span>');
  });
});

Can I disable SCE completely?

Yes, you can. However, this is strongly discouraged. SCE gives you a lot of security benefits for little coding overhead. It will be much harder to take an SCE disabled application and either secure it on your own or enable SCE at a later stage. It might make sense to disable SCE for cases where you have a lot of existing code that was written before SCE was introduced and you're migrating them a module at a time.

That said, here's how you can completely disable SCE:

angular.module('myAppWithSceDisabledmyApp', []).config(function($sceProvider) {
  // Completely disable SCE.  For demonstration purposes only!
  // Do not use in new projects.
  $sceProvider.enabled(false);
});

Usage

$sce();

Methods