Subjective? No, not really.
The basic ethical principle is to not interfere with another's well-being. The definition of well-being may appear changeable but it should be the subject's definition and standard of well-being, not your own.
From the single principle, which we may restate as "treat others the way they want to be treated," we can derive prohibitions against murder and assault; lying, and from that cheating; theft, and from that malicious vandalism (depriving property from another, even if you don't benefit from it); and so on.
The relationship goes both ways. The subject may not turn it around and complain that your existence interferes with their well-being, since ending your existence would interfere with your well-being.
These derivations from the core principle are not subjective, and in fact are quite logical and can be reproduced by anyone regardless of culture.
Contrast that with morality, which is very subjective. Moral codes are defined by your culture, and will vary from time to time and place to place, and even from sect to sect within the greater culture.
Moral codes may be based on ethical principles but are not necessarily so, and not every moral edict is derived from an ethical principle or derivation. Your culture may have conditioned you to think that certain practices ethical but, when turned around and viewed from a subject's point of view clearly are not. The aforementioned issue of slavery is like that: the slave-owners may have decided that slavery is conducted with the slave's best interest at heart, but ask the slave how s/he feels about it and you'll get a very different answer.