More than a third of UK current account holders will be able to use Zapp to pay directly from their accounts without cash or card.
The multinational bank confirms attack, saying it "did not affect any customer data, but did prevent customers using HSBC online services."
HSBC banking appears to be offline, at cash points, over the phone, in shops and online.
Such a move to replace the BlackBerry--still in the mulling stages--could see the Apple gadget "potentially be deployed, conservatively, to 200,000 people."
Customers of HSBC, Bank of America, and Washington Mutual suffer highest rates of identity theft in the banking industry, according to report by a UC Berkeley Law School researcher.
Nine banks have signed up to Paym, which allows you to send money to someone from your phone knowing only their phone number.
A new mobile payments service launches in the UK this Spring to send and receive money without knowing bank details.
Two analysts recently raised their rating on the smartphone maker, citing high hopes for sales of the HTC One.
The company hasn't said exactly what kind of issues it's seeing, and there has been no disruption or denial-of-service attacks.