First-authorship was taken by my Master's supervisor; how to strengthen my PhD application?
(I assume here that you're talking about admission to PhD programs.)

I'm not in computer science, so I can't speak with authority.  We have a number of users here who are in US computer science departments and maybe one of them can step in.

However, I would think that first-author publications would not be essential for admission to good PhD programs in the US.  Keep in mind that in the US system, most students would enter a PhD program immediately after their undergraduate (bachelor's) degree, without doing a masters first.  These students would not necessarily have any publications at all.  The admission committee would expect somewhat more from a student with a masters, but I'd think any publications, first-author or not, would be a pretty good sign of your research ability and preparation to start a PhD.

Another thing to note is that recommendation letters are very important for admission to US PhD programs.  If your advisor writes you a letter which speaks highly of the work you did, that should help your application a lot.