4.8.2 The iframe element

Categories:
Flow content.
Phrasing content.
Embedded content.
Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
Text that conforms to the requirements given in the prose.
Content attributes:
Global attributes
src
srcdoc
name
sandbox
seamless
width
height
DOM interface:
interface HTMLIFrameElement : HTMLElement {
           attribute DOMString src;
           attribute DOMString srcdoc;
           attribute DOMString name;
  [PutForwards=value] readonly attribute DOMSettableTokenList sandbox;
           attribute boolean seamless;
           attribute DOMString width;
           attribute DOMString height;
  readonly attribute Document? contentDocument;
  readonly attribute WindowProxy? contentWindow;
};

The iframe element represents a nested browsing context.

The src attribute gives the address of a page that the nested browsing context is to contain. The attribute, if present, must be a valid non-empty URL potentially surrounded by spaces.

If the itemprop is specified on an iframe element, then the src attribute must also be specified.

The srcdoc attribute gives the content of the page that the nested browsing context is to contain. The value of the attribute is the source of an iframe srcdoc document.

For iframe elements in HTML documents, the attribute, if present, must have a value using the HTML syntax that consists of the following syntactic components, in the given order:

  1. Any number of comments and space characters.
  2. Optionally, a DOCTYPE.
  3. Any number of comments and space characters.
  4. The root element, in the form of an html element.
  5. Any number of comments and space characters.

For iframe elements in XML documents, the attribute, if present, must have a value that matches the production labeled document in the XML specification. [XML]

If the src attribute and the srcdoc attribute are both specified together, the srcdoc attribute takes priority. This allows authors to provide a fallback URL for legacy user agents that do not support the srcdoc attribute.

When an iframe element is inserted into a document, the user agent must create a nested browsing context, and then process the iframe attributes for the first time.

When an iframe element is removed from a document, the user agent must discard the nested browsing context.

This happens without any unload events firing (the nested browsing context and its Document are discarded, not unloaded).

Whenever an iframe element with a nested browsing context has its srcdoc attribute set, changed, or removed, the user agent must process the iframe attributes.

Similarly, whenever an iframe element with a nested browsing context but with no srcdoc attribute specified has its src attribute set, changed, or removed, the user agent must process the iframe attributes.

When the user agent is to process the iframe attributes, it must run the first appropriate steps from the following list:

If the srcdoc attribute is specified

Navigate the element's browsing context to a resource whose Content-Type is text/html, whose URL is about:srcdoc, and whose data consists of the value of the attribute. The resulting Document must be considered an iframe srcdoc document.

If the src attribute is specified but the srcdoc attribute is not

  1. If the value of the src attribute is the empty string, jump to the empty step below.

  2. Resolve the value of the src attribute, relative to the iframe element.

  3. If that is not successful, then jump to the empty step below.

  4. If the resulting absolute URL is an ASCII case-insensitive match for the string "about:blank", and the user agent is processing this iframe's attributes for the first time, then jump to the empty step below. (In cases other than the first time, about:blank is loaded normally.)

  5. Navigate the element's browsing context to the resulting absolute URL.

Empty: When the steps above require the user agent to jump to the empty step, if the user agent is processing this iframe's attributes for the first time, then the user agent must queue a task to fire a simple event named load at the iframe element. (After jumping to this step, the above steps are not resumed.) No load event is fired at the about:blank document itself.

Otherwise

Queue a task to fire a simple event named load at the iframe element.

Any navigation required of the user agent in the process the iframe attributes algorithm must be completed as an explicit self-navigation override and with the iframe element's document's browsing context as the source browsing context.

Furthermore, if the browsing context's session history contained only one Document when the process the iframe attributes algorithm was invoked, and that was the about:blank Document created when the browsing context was created, then any navigation required of the user agent in that algorithm must be completed with replacement enabled.

If, when the element is created, the srcdoc attribute is not set, and the src attribute is either also not set or set but its value cannot be resolved, the browsing context will remain at the initial about:blank page.

If the user navigates away from this page, the iframe's corresponding WindowProxy object will proxy new Window objects for new Document objects, but the src attribute will not change.

Here a blog uses the srcdoc attribute in conjunction with the sandbox and seamless attributes described below to provide users of user agents that support this feature with an extra layer of protection from script injection in the blog post comments:

<article>
 <h1>I got my own magazine!</h1>
 <p>After much effort, I've finally found a publisher, and so now I
 have my own magazine! Isn't that awesome?! The first issue will come
 out in September, and we have articles about getting food, and about
 getting in boxes, it's going to be great!</p>
 <footer>
  <p>Written by <a href="/users/cap">cap</a>, 1 hour ago.
 </footer>
 <article>
  <footer> Thirteen minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>did you get a cover picture yet?"></iframe>
 </article>
 <article>
  <footer> Nine minutes ago, <a href="/users/cap">cap</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>Yeah, you can see it <a href=&quot;/gallery?mode=cover&amp;amp;page=1&quot;>in my gallery</a>."></iframe>
 </article>
 <article>
  <footer> Five minutes ago, <a href="/users/ch">ch</a> wrote: </footer>
  <iframe seamless sandbox srcdoc="<p>hey that's earl's table.
<p>you should get earl&amp;amp;me on the next cover."></iframe>
 </article>

Notice the way that quotes have to be escaped (otherwise the srcdoc attribute would end prematurely), and the way raw ampersands (e.g. in URLs or in prose) mentioned in the sandboxed content have to be doubly escaped — once so that the ampersand is preserved when originally parsing the srcdoc attribute, and once more to prevent the ampersand from being misinterpreted when parsing the sandboxed content.

In the HTML syntax, authors need only remember to use U+0022 QUOTATION MARK characters (") to wrap the attribute contents and then to escape all U+0022 QUOTATION MARK (") and U+0026 AMPERSAND (&) characters, and to specify the sandbox attribute, to ensure safe embedding of content.

Due to restrictions of the XHTML syntax, in XML the U+003C LESS-THAN SIGN character (<) needs to be escaped as well. In order to prevent attribute-value normalization, some of XML's whitespace characters — specifically U+0009 CHARACTER TABULATION (tab), U+000A LINE FEED (LF), and U+000D CARRIAGE RETURN (CR) — also need to be escaped. [XML]

The name attribute, if present, must be a valid browsing context name. The given value is used to name the nested browsing context. When the browsing context is created, if the attribute is present, the browsing context name must be set to the value of this attribute; otherwise, the browsing context name must be set to the empty string.

Whenever the name attribute is set, the nested browsing context's name must be changed to the new value. If the attribute is removed, the browsing context name must be set to the empty string.

When content loads in an iframe, after any load events are fired within the content itself, the user agent must queue a task to fire a simple event named load at the iframe element. When content whose URL has the same origin as the iframe element's Document fails to load (e.g. due to a DNS error, network error, or if the server returned a 4xx or 5xx status code or equivalent), then the user agent must queue a task to fire a simple event named error at the element instead. (This event does not fire for parse errors, script errors, or any errors for cross-origin resources.)

The task source for these tasks is the DOM manipulation task source.

A load event is also fired at the iframe element when it is created if no other data is loaded in it.

When there is an active parser in the iframe, and when anything in the iframe is delaying the load event of the iframe's browsing context's active document, the iframe must delay the load event of its document.

If, during the handling of the load event, the browsing context in the iframe is again navigated, that will further delay the load event.

The sandbox attribute, when specified, enables a set of extra restrictions on any content hosted by the iframe. Its value must be an unordered set of unique space-separated tokens that are ASCII case-insensitive. The allowed values are allow-forms, allow-popups, allow-same-origin, allow-scripts, and allow-top-navigation. When the attribute is set, the content is treated as being from a unique origin, forms and scripts are disabled, links are prevented from targeting other browsing contexts, and plugins are secured. The allow-same-origin keyword allows the content to be treated as being from the same origin instead of forcing it into a unique origin, the allow-top-navigation keyword allows the content to navigate its top-level browsing context, and the allow-forms, allow-popups and allow-scripts keywords re-enable forms, popups, and scripts respectively.

Setting both the allow-scripts and allow-same-origin keywords together when the embedded page has the same origin as the page containing the iframe allows the embedded page to simply remove the sandbox attribute.

Sandboxing hostile content is of minimal help if an attacker can convince the user to just visit the hostile content directly, rather than in the iframe. To limit the damage that can be caused by hostile HTML content, it should be served from a separate dedicated domain.

While the sandbox attribute is set or changed, the user agent must parse the sandboxing directive using the attribute's value as the input and the iframe element's nested browsing context's iframe sandboxing flag set as the output.

These flags only take effect when the nested browsing context of the iframe is navigated. Removing them, or removing the entire sandbox attribute, has no effect on an already-loaded page.

In this example, some completely-unknown, potentially hostile, user-provided HTML content is embedded in a page. Because it is served from a separate domain, it is affected by all the normal cross-site restrictions. In addition, the embedded page has scripting disabled, plugins disabled, forms disabled, and it cannot navigate any frames or windows other than itself (or any frames or windows it itself embeds).

<p>We're not scared of you! Here is your content, unedited:</p>
<iframe sandbox src="http://usercontent.example.net/getusercontent.cgi?id=12193"></iframe>

It is important to use a separate domain so that if the attacker convinces the user to visit that page directly, the page doesn't run in the context of the site's origin, which would make the user vulnerable to any attack found in the page.

In this example, a gadget from another site is embedded. The gadget has scripting and forms enabled, and the origin sandbox restrictions are lifted, allowing the gadget to communicate with its originating server. The sandbox is still useful, however, as it disables plugins and popups, thus reducing the risk of the user being exposed to malware and other annoyances.

<iframe sandbox="allow-same-origin allow-forms allow-scripts"
        src="http://maps.example.com/embedded.html"></iframe>

Suppose a file A contained the following fragment:

<iframe sandbox="allow-same-origin allow-forms" src=B></iframe>

Suppose that file B contained an iframe also:

<iframe sandbox="allow-scripts" src=C></iframe>

Further, suppose that file C contained a link:

<a href=D>Link</a>

For this example, suppose all the files were served as text/html.

Page C in this scenario has all the sandboxing flags set. Scripts are disabled, because the iframe in A has scripts disabled, and this overrides the allow-scripts keyword set on the iframe in B. Forms are also disabled, because the inner iframe (in B) does not have the allow-forms keyword set.

Suppose now that a script in A removes all the sandbox attributes in A and B. This would change nothing immediately. If the user clicked the link in C, loading page D into the iframe in B, page D would now act as if the iframe in B had the allow-same-origin and allow-forms keywords set, because that was the state of the nested browsing context in the iframe in A when page B was loaded.

Generally speaking, dynamically removing or changing the sandbox attribute is ill-advised, because it can make it quite hard to reason about what will be allowed and what will not.

Potentially hostile files should not be served from the same server as the file containing the iframe element. Using a different domain ensures that scripts in the files are unable to attack the site, even if the user is tricked into visiting those pages directly, without the protection of the sandbox attribute.

If the allow-scripts keyword is set along with allow-same-origin keyword, and the file is from the same origin as the iframe's Document, then a script in the "sandboxed" iframe could just reach out, remove the sandbox attribute, and then reload itself, effectively breaking out of the sandbox altogether.

The seamless attribute is a boolean attribute. When specified, it indicates that the iframe element's browsing context is to be rendered in a manner that makes it appear to be part of the containing document (seamlessly included in the parent document). Specifically, when the attribute is set on an iframe element whose owner Document's active sandboxing flag set does not have the sandboxed seamless iframes flag set, and while either the browsing context's active document has the same origin as the iframe element's document, or the browsing context's active document's address has the same origin as the iframe element's document, or the browsing context's active document is an iframe srcdoc document, the following requirements apply:

If the attribute is not specified, or if the origin conditions listed above are not met, then the user agent should render the nested browsing context in a manner that is clearly distinguishable as a separate browsing context, and the seamless browsing context flag must be set to false for that browsing context.

It is important that user agents recheck the above conditions whenever the active document of the nested browsing context of the iframe changes, such that the seamless browsing context flag gets unset if the nested browsing context is navigated to another origin.

The attribute can be set or removed dynamically, with the rendering updating in tandem.

In this example, the site's navigation is embedded using a client-side include using an iframe. Any links in the iframe will, in new user agents, be automatically opened in the iframe's parent browsing context; for legacy user agents, the site could also include a base element with a target attribute with the value _parent. Similarly, in new user agents the styles of the parent page will be automatically applied to the contents of the frame, but to support legacy user agents authors might wish to include the styles explicitly.

<nav><iframe seamless src="nav.include.html"></iframe></nav>

The iframe element supports dimension attributes for cases where the embedded content has specific dimensions (e.g. ad units have well-defined dimensions).

An iframe element never has fallback content, as it will always create a nested browsing context, regardless of whether the specified initial contents are successfully used.

Descendants of iframe elements represent nothing. (In legacy user agents that do not support iframe elements, the contents would be parsed as markup that could act as fallback content.)

When used in HTML documents, the allowed content model of iframe elements is text, except that invoking the HTML fragment parsing algorithm with the iframe element as the context element and the text contents as the input must result in a list of nodes that are all phrasing content, with no parse errors having occurred, with no script elements being anywhere in the list or as descendants of elements in the list, and with all the elements in the list (including their descendants) being themselves conforming.

The iframe element must be empty in XML documents.

The HTML parser treats markup inside iframe elements as text.

The IDL attributes src, srcdoc, name, sandbox, and seamless must reflect the respective content attributes of the same name.

The contentDocument IDL attribute must return the Document object of the active document of the iframe element's nested browsing context, if any, or null otherwise.

The contentWindow IDL attribute must return the WindowProxy object of the iframe element's nested browsing context, if any, or null otherwise.

Here is an example of a page using an iframe to include advertising from an advertising broker:

<iframe src="http://ads.example.com/?customerid=923513721&amp;format=banner"
        width="468" height="60"></iframe>