Imagine that you have a simple site with only 2 pages: login.aspx and secret.aspx. Your site is secured using nothing but ASP.net forms authentication and an ASP.net Login serv
You still have some variables that aren't accounted for:
Not all of those issues are MS specific, but they're worth mentioning because any of them could easily outweigh the issue you're asking about, if not taken care of. But, for the purpose of your question I'll assume there aren't any problems with them.
In that case, I'm pretty sure the forms authentication does what it's supposed to do. I don't think there's any currently active exploit out there.