Quote:Mozilla is testing a new security feature in Firefox Nightly currently that adds rel="noopener" automatically to links that use target="_blank".

Target="_blank" instructs browsers to open the link target in a new tab in the web browser automatically; without the target attribute, links would open in the same tab unless users use built-in browser functionality, e.g. by holding down Ctrl or Shift, to open the link in a different way.

Rel="noopener is supported by all major web browsers. The attribute makes sure that window-opener is null in modern browsers. Null means that it contains no value.

If rel="noopener" is not specified, linked resources have full control over the originating window object even if the resources are on different origins. The destination link could manipulate the originating document, e.g. replace it with a lookalike for phishing, display advertisement on it or manipulate it in any other way imaginable.

You can check out a demo page on rel="noopener" abuse here. It is harmless but highlights how destination sites may alter the originating site if the attribute is not used.

Full reading: https://www.ghacks.net/2018/11/30/firefox-security-relnoopener-for-target_blank/