Use of Web Link to Untrusted Target with window.opener Access

Description

When a user clicks a link to an external site ("target"), the target="_blank" attribute causes the target site's contents to be opened in a new window or tab, which runs in the same process as the original page. The window.opener object records information about the original page that offered the link. If an attacker can run script on the target page, then they could read or modify certain properties of the window.opener object, including the location property - even if the original and target site are not the same origin. An attacker can modify the location property to automatically redirect the user to a malicious site, e.g. as part of a phishing attack. Since this redirect happens in the original window/tab - which is not necessarily visible, since the browser is focusing the display on the new target page - the user might not notice any suspicious redirection.

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In this example, the application opens a link in a named window/tab without taking precautions to prevent the called page from tampering with the calling page's location in the browser.

There are two ways that this weakness is commonly seen. The first is when the application generates an <a> tag is with target="_blank" to point to a target site:

<a href="http://attacker-site.example.com/useful-page.html" target="_blank">

If the attacker offers a useful page on this link (or compromises a trusted, popular site), then a user may click on this link. However, the attacker could use scripting code to modify the window.opener's location property to redirect the application to a malicious, attacker-controlled page - such as one that mimics the look and feel of the original application and convinces the user to re-enter authentication credentials, i.e. phishing:

window.opener.location = 'http://phishing.example.org/popular-bank-page';

To mitigate this type of weakness, some browsers support the "rel" attribute with a value of "noopener", which sets the window.opener object equal to null. Another option is to use the "rel" attribute with a value of "noreferrer", which in essence does the same thing.

<a href="http://attacker-site.example.com/useful-page.html" target="_blank" rel="noopener noreferrer">

A second way that this weakness is commonly seen is when opening a new site directly within JavaScript. In this case, a new site is opened using the window.open() function.

var newWindow = window.open("http://attacker-site.example.com/useful-page.html", "_blank");

To mitigate this, set the window.opener object to null.

var newWindow = window.open("http://attacker-site.example.com/useful-page.html", "_blank");
newWindow.opener = null;

See Also