I could be wrong, but it seems to me that the very process of canning the extract would take care of any spores.
Outside of that, it's been quite some time since I brewed from extract but do remember doing a batch many years ago using extract that was past it's prime. I'm still here to tell about it. The resulting beer, however, was definitely sub par. Safe to say that old extract may not kill you, but will almost guarantee a finished beer with some off flavors.