My co-supervisor wants authorship in a paper he didn't contribute I have two supervisors, in different countries, one in home institute who sent me as an exchange student to the host institute, where I closely worked with another co-supervisor who significantly contributed to my work, taking place of the last author in the conference paper I have submitted (not published). He reviewed my work, my paper and also was a really good advisor. My other supervisor, one from the home country had nothing to do with my project except on record. His domain is completely different from what I have worked on. 

Now, in my master's thesis that I presented in the home institute in front of my co-supervisor and other faculty in the institute, I mentioned that I've submitted a paper based on the work I had done. I was questioned about the authorship of this co-supervisor, and was told it is unethical to leave him out of authorship, because by default, the supervisor must get the last authorship. 

Now I fear that due to ego issues, they will reduce my grades because I didn't mention this co-supervisor's name in the paper. 

How do you suggest I tackle this situation and am I wrong to leave him out? His only contribution was to provide me recommendation for sending me to the exchange program, and review my final thesis and presentation which I suppose every supervisor must do, whether or not a paper is published. 

P.S. In my institute, having a paper published gets us better grades and hence I decided to mention about the submission. But now it looks like everything is going to take a U-turn on me.
Most journals have authorship policies. These are recommended by the International Committee of Medical Journal Editors (http://www.icmje.org/recommendations/browse/roles-and-responsibilities/defining-the-role-of-authors-and-contributors.html), among others.

The Committee on Publication Ethics (COPE) has guidance on how to avoid authorship disputes. https://publicationethics.org/files/2003pdf12_0.pdf

Be factual with your supervisor. Provide the authorship guidelines and ask him to list his contribution to the research article. You can explain that the culture on authorship has changed and that automatic authorship for higher level supervisors is no longer considered appropriate by publishers (as evidenced by the guidelines of the publication). Truthfully, it would be best if your host supervisor perform this deed because they are on an equal footing and apparently had some prior connection. Clearly, it puts you in a difficult situation.