Even though this vulnerability is rated by many bug bounty programs as a low severity bug, it is worth looking for it as it is very easy to find.
When tabnabbing, the attacker searches for links that are inserted into the website and are under his control. Such links may be contained in a forum post, for example. Once he has found this kind of functionality, it checks that the link's rel attribute does not contain the value noopener and the target attribute contains the value _blank. If this is the case, the website is vulnerable to tabnabbing.
1. Attacker posts a link to a website under his control that contains the following JS code: window.opener.location = "http://evil.com"
2. He tricks the victim into visiting the link, which is opened in the browser in a new tab.
3. 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.
4. If the victim opens the background tab again and doesn't look at the address bar, it may happen that he thinks he is logged out, because a login page appears, for example.
5. The victim tries to log on again and the attacker receives the credentials
As already mentioned, you have to search for the following link formats:
<a href="..." target="_blank" rel="" />
or
<a href="..." target="_blank" />