Whoops! File Found. This is Not a 404
A crypto 404, or more commonly known as a soft 404, is an outputted error page that gives a 200 (“OK” File Found) response code instead of the correct response code of 404 (“NOT FOUND”).
Search engines in particular look at response codes to assess whether they should be included in their indices.
With crypto 404s, however, and the erroneous 200 OK status that they send, search engines have to work out if they are actual files of interest or files outputted in error. With some search engines this can sometimes result in a duplication issue as they will assume that the soft 404s are files of interest and all having the same content.
Where does the term come from?
The origination of the term Crypto 404 is unclear but we do know that Google has been using the term internally for a good number of years, pre-2006. Crypto coming from the Greek kruptos which means “hidden, concealed, secret“.
How is a crypto 404 (soft 404) created?
Initially it is created from a response code that gives a 200 OK instead of a 404 NOT FOUND when visiting a page or file that should not exist. A response code is a tiny bit of information that is sent from the server in the header of the file.
This typically happens from either having a misconfigured server, an out of date or poorly maintained CMS (Content Management System), or custom code that has been wrongly implemented which is neglecting the 404 altogether.
How do you correct them?
Most search engines suggest that these type of non-404s be corrected and give the appropriate 404/410 response codes in its place. Google has a lot of resources on how to resolve them and you can check if your own site has any crawling issues through using their own Webmaster Tools.
Matt Cutts on “Not Found” pages that do not return a 404
Below Matt Cutts, head of the Webspam team in the Search Quality group at Google, explains how Google handles the pages that do not return a 404 correctly.