High external linking issue means that a specific URL contains numerous links to third-part websites.
The importance of the issue
It is considered to be a good practice to refer to third-party reputable websites for a comprehensive topic coverage of a specific page, or backlink a source of information. Nevertheless, employment of too many external links might make a search engine treat a page as spam, therefore having a negative impact over the entire website.
John Mueller, a Google employee, tells about outbound links:
“I would watch out for a few types of links, though. In particular, if you’re linking out to a site because of an arrangement like “You link to me and I’ll link to you.”
Or because it’s an advertisement or if it’s being done in your site’s comments and you’re not really sure how good those links are.
In all of these situations your pages aren’t linking out because you think the other page is a great match for users.
For these kinds of links we recommend using the rel nofollow link attribute.
In the most common cases however if you’re linking out naturally from your content to other sites that offer additional value and more context then that’s fine and there’s nothing special that you need to do.
How to check the issue
You can browse external links from your page using the Links column in Sitechecker
How to fix this issue
Study all external links at a current page. Leave only those links that can actually help a user or point to the content author. Check if there are any links you haven’t set. The presence of such links, as well as links to 18+websites, might indicate that your website has been hacked.