It depends on the tone of voice, the situation, and the words that follow. It is a signal saying "Pay attention." It is "brusque." It may or may not be "rude."

"Hey!" by itself, in a loud voice, to a stranger, is intended to get attention as quickly as possible, without regard to politeness. It is either a warning ("Hey! Watch out! You're going the wrong way on a one-way street!) or an expression of indignation ("Hey! What are you doing? I was here before you.")

If I were lost in an airport and people were streaming by me, I can imagine saying to someone "Hey! Excuse me, do you happen to know which way it is to Terminal C?" The "Hey!" gets attention, the "excuse me" makes it polite.

I think there are many better options than just "hey". I notice some learners confuse casual speech for friendly speech, so their friendly(?) greeting comes across as too familiar and even disrespectful.  It's much easier to use the standard greetings instead.

There's nothing wrong with being polite at your first meeting - don't you agree? :)

If anyone thinks that "hey" is a proper, respectful form of greeting, then try this:

The next time you go to a job interview, greet the interviewers by saying "Hey, nice to meet you" or "Hey, thanks for interviewing me today."

I can almost guarantee that:

1...  the interview will not last vey long, and

2...  the interview will not result in a job offer.

"hey" is an interjection, a word used to get someone's attention, sometimes in a way that is not very polite:

I agree with several other comments, including Dan Smith’s and Peachey’s.
To me it’s not so much a matter of rudeness as it’s a matter of respecting another person’s personal space (or is that the same thing? - I don’t know). I would generally not like if a stranger said “hey” to me. It’s too friendly/familiar. It indicates that the person doesn’t respect my personal boundaries and that will make me feel a little uncomfortable.
Of course, if I can easily tell that the person is not a native speaker of English I will not expect that person to know that. I’m talking about how I would feel if a native speaker, who is familiar with the cultural norms of English speaking countries, approached me and said “hey” to me.

I don't have a problem with "hi". To me it's not as familiar/casual as "hey".