[chrismcb]

My first thought when I read the title was "no, don't leave. Working on legacy code will teach you the number one thing about programming, that no one else will teach you... Writing good code is about writing maintainable code." It is hard to fix hacky code, but do your best to not write hacky code. Another lesson to learn... To ahead and learn more about the attack they are using. Yes, you may never use that attack again, but it will actually make it easier to learn a new stack later on. 20 years is really not all that ancient either. JavaScript is over 20 years old... It might help you to learn some of the older technologies. Now, that being said, I don't know the extent you were promised to work on new things, and how much you were lied to and how bad the culture is.

I suggest you start looking for a new job, but in the meantime work hard on your current job. Take pride in your work. No matter where you go there will always be a legacy system you will have to work with. New or different technologies to learn.