Except, individuals don't own accounts at DTC, so you'll have to hack and twiddle a few bits here and there of the custodial bank as well. But these are reconciled each day against Security Position Reports generated at the end of the day, so your bits need to reconcile with trades. So you need to hack and twiddle a few bits here and there for a chosen counterparty as well. Except, the exchanges keep track of trades too, so you'll have to twiddle a few bits here and there for an exchange as well.

It's not as simple as this at all, which is why the system has existed for decades. It's not the most efficient, but it's not something that will be replaced by a hackathon project either.

Tom Clancy used an attack as part of the plot of "Debt of Honor".