WebTechee

Smart External Links Manager detects external links in post content and adds:
<ul>
<li>target=”_blank”</li>
<li>rel=”noopener noreferrer”</li>
</ul>
Internal links, anchor links, mailto, and tel links are ignored. 
This plugin is intentionally lightweight and works out of the box without configuration.

Webtechee External Links Security

Automatically adds secure attributes to external links in post content.

Tran Ngoc Tuan Anh

This plugin adds rel=”nofollow” to all external links in post content. So, your flow of PageRank can not run to other websites and your blog’s SEO is improved.
The internal links are unchanged.
Requires PHP5.

Webtechee External Links Security vs External Nofollow