Recent postings at Webmaster World indicate that Yahoo has been or might be misinterpreting http and https and considering them to be sub domains. The issue seems to be specifically related to websites that have some CGI (Common Gateway Interface) scripts and use https. The result is that the webmasters are losing their ranking for main target keyword terms.
There are quite a few postings at Webmaster World that give an insight into the problem. Some interesting ones are mentioned below:
As a result of this Yahoo seems to be applying a dupe penalty and my site, which used to be at #1 is now around #50 for the main target keyword term.
Is there a simple 301 redirect solution that will sort this out without harming my ranking elsewhere, I'm #2 on Google for this term, or am I better off allowing Yahoo to wither on the vine?"
"I've had this exact same problem for over a year, I've also got a robot text file on my Https site that denies indexing yet Yahoo still indexes the https as a sub-domain.
As for a redirect i have a server wide 301 redirect from non www to www for years now and still Yahoo indexes some of my url's with the non www prefix. Yahoo's capability of displaying and following 301 redirects is very poor in my experience and they seem to display url's from links rather than what the server is telling the Bot to do.
try and locate the offending page that links in to the https site is the only solution that i can suggest.
I'd greatly appreciate any help if anyone else has some detailed information on how to remove these as Yahoo has a strong following still in some parts of the world."
"There's a related thread in the Google Search forum right now, with a good approach that would work for all search engines. It's not just Yahoo that treats different protocols as different URLs when they both resolve.
Hopefully Yahoo will fix the problem soon and save the webmasters from further inconvenience and damage to their rankings.