WordPress attempts to guess a redirect URL for a 404 request

Today I found out that one of the client’s WordPress sites redirects to a post page for a request that is supposed to return a 404 page. The request to https://example.com/a brought you to the https://example.com/about page.

At first, I thought this came from a redirection plugin or Yoast SEO Premium plugin. However, there was no redirection on any of those plugins.

Investigating response headers

Because none of the plugins defined the redirection, I opened the web developer tools on the Google Chrome I used. I clicked the Network tab and checked the response headers for the request to https://example.com/a

Here’s the result that I filtered:

location: https://example.com/about
x-redirect-by: WordPress

That x-redirect-by showed WordPress itself did the redirect. If another plugin like Yoast does it, its value would be:

x-redirect-by: Yoast SEO Premium

Well, here is the link to the documentation where WordPress introduces this header: https://core.trac.wordpress.org/ticket/42313

It turns out to be WordPress redirection

To prove it’s the WordPress that does the redirect, I tried the /a request on the other client WordPress sites. Those other sites gave me the same redirection.

Conclusion

I finally figured out that since WordPress 5.0.0, there’s a function that attempts to guess the correct URL for a 404 request. Here is the official developer documentation that mentions it: https://developer.wordpress.org/reference/functions/redirect_guess_404_permalink/.