"Fucking up the banks' servers / sites" would also harm all of its banking customers, would it not? My best guess would be that this would not be allowed, but I've been wrong before so if you truly feel like you can make an argument for how it would help people rather than harm them (which I'm not sure is even possible in this scenario) then you can certainly make your case and check with DPR on it. That said, you wouldn't just be causing the banks a headache - you'd be interrupting service to customers who did nothing to deserve their money or accounts being inaccessible or compromised.