I don't think humans are 100% extinction-proof, however, I do think we could survive a lot of things that most other species could not. Besides, there are billions of us. Even if you whittle that down to a few thousand, that is a survivable number of people to prevent extinction.
I think our ultimate limit, given current technology, would be the destruction of our atmosphere (that is, making it unbreathable by us.) Even then, I suppose we could make indoor environments and grow plants. However, none of the Biosphere projects actually panned out, so it's debatable whether we could really survive this way long-term.