A financial services industry organization and several cyber security companies said a group of "hactivists" that impeded access to some major U.S. online banking sites in September had so far failed to gain traction in a second campaign that began this week.
"There has been no impact to critical financial systems," the Financial Services Information Sharing and Analysis Center, or FS-ISAC, said in a statement.
Tom Kellermann, vice president of cyber security at security software maker Trend Micro Inc, said the banks had been "very successful.
"I wouldn't see any prolonged outages for the major financial institutions in the coming year."
Arbor Networks, which sells equipment that companies can use to protect their websites against attack, said banks had bolstered their defenses since the last round of attacks in September.
"Everyone has been through this once or twice," said Dan Holden, an Arbor director. "There is just a better level of preparedness."