American Express and Dwolla suffered website outages, which the companies attributed to distributed denial-of-service (DDOS) attacks. Such attacks are designed to knock out a website by flooding it with more traffic than it is designed to handle.

American Express' site experienced a DDOS attack for about two hours on this afternoon, the company says.

"We experienced intermittent slowing on our website that would have disrupted customers' ability to access their account information," said Amelia Woltering, an American Express spokeswoman, in an email. "We had a plan in pace to defend against a potential attack and have taken steps to minimize ongoing customer impact."

There is no evidence that customer data was compromised, Woltering said. Amex did not comment on the source of the attack.

Dwolla, in a blog post today, says it has been the target of "distributed denial of service event, resulting in limited or no availability to the website, Dwolla.com," since March 27.

The Amex and Dwolla websites were both operational as of late today.

Web-focused companies can defend against such attacks. In 2011, PayPal weathered a DDOS attack when other payment brands saw their websites blocked. The 2011 attacks were in protest to the payment companies' decisions to stop handling payments for WikiLeaks.

One defense is to use cloud computing to increase bandwidth during such an attack.

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