Banks can only cross their fingers and hope the defenses they have in place can withstand cyberattacks like the one that disrupted the online banking site of Wells Fargo & Co., experts say.On Tuesday, Wells Fargo became the latest victim in a string of attacks on banks that started last week at J.P. Morgan Chase & Co. and Bank of America Corp. \u00a0A group calling itself "Mrt. Izz ad-Din al-Qassam Cyber Fighters" claimed responsibility and threatened to hit U.S. Bankcorp and PNC Financial Services Group on Thursday, said a post on Pastebin.The latest attack took down Wells Fargo's online banking site intermittently. The bank apologized for the downtime on Twitter and appeared to be back up on Wednesday.Wells Fargo has not provided details of the attack, which appeared in media reports to be a distributed denial of service attack, or DDoS. Dmitri Alperovitch, chief executive of CrowdStrike, a private security firm investigating the attacks, told The Wall Street Journal, "The amount of bandwidth that is flooding the websites is very large, much larger than in other attacks, and in a sense unprecedented."DDoS attacks are crude but effective. The attackers typically use large networks of compromised computers to overwhelm a website with requests. "I would say few organizations can face a full on botnet DDoS attack and stay operational," said Brent Huston, chief executive of MicroSolved.[Related stories:\u00a0Islamic hacktivists' bank attack claims gain credibility |\u00a0Wells Fargo recovers after site outage\u00a0|\u00a0Theories mount on bank attacks, but experts stress defense\u00a0|\u00a0Arab hackers attack Western websites over film\u00a0|\u00a0Best defense against cyberattacks is good offense, says former DHS official]The battle often comes down to the amount of bandwidth a banking site has and whether it is large enough to withstand traffic from the botnet and customers. "If the attacker can find away to exhaust the resources of any business critical component of the system, they win," said Jeremiah Grossman, chief technology officer for Whitehat Security.Beyond increasing bandwidth, banks have other defensive options, such as redirecting traffic to a cloud-based alternative site or using alternate routing tools to move traffic to other locations to balance the load. More expensive options include appliances that do packet analysis to separate good and bad traffic, sending the latter to an unused IP address.\u00a0"Those [tools] are well beyond the reach of most small to mid-size organizations," Huston said. "Many banks and credit unions in the small to mid-size range size their bandwidth to basically handle their average web traffic. They try and save money by matching their expenditures to their average use, thus, they get caught in the vice-like squeeze of cost versus risk of a DDoS when these things come along."Banks the size of Wells Fargo typically have the strongest Internet security. "By and large, most US banks have invested pretty heavily in Internet security," said Andrew Plato, chief executive and president of Anitian Enterprise Security. "It would be hard to imagine that a company with the size and reputation of Wells Fargo having bad Internet security."The possibility of a site being downed by a DDoS attack is a fact of life on the Internet and users have to be willing to accept it. "That's just the neighborhood, unfortunately," said Dan Olds, an analyst for Gabriel Consulting Group.To lessen customer anger, banks should send notices when a disruption occurs and how long the site will be inaccessible. "That's great customer service," Olds said.The group that claimed to be behind the Wells Fargo attack indicated it was in retaliation for the video trailer denigrating the Prophet Muhammad. The amateurish YouTube video made in the U.S. has sparked violent protests in the Middle East and other regions.