BTC is not, itself, a security.  However, there are people creating and buying and selling items packaged as securities in which the underlying business is BTC-based.  They trade on what are purported to be "stock exchanges."  Some of these are outright Ponzi scams.

Similarly, BTC is not, itself, a Ponzi scam.  However, Ponzi scams exist in which the underlying currency is BTC.

If nothing else, some of these highly questionable products look like some kind of commodities future.