Schwab Website Hit by Cyberattack

Charles Schwab
Adam Jeffery | CNBC

Charles Schwab's website was working again Tuesday evening after an outage that left all of its online customers unable to make trades for about one hour and 40 minutes.

Schwab believes the disruption was caused by a denial-of-service attack. The site was hit by a high volume of web traffic just before regular U.S. stock trading closed at 4 pm ET.

There has been an increase in attacks that have temporarily disabled financial websites.

(Read more: Bank Website Attacks Reach New Highs)

During the six weeks ending March 31, U.S. bank websites were offline for a total of 249 hours. During the same period in 2012, the same websites were down for 140 hours, according to an analysis by Keynote Systems, a company that checks website availability every five minutes.

While a Keynote spokesman said it doesn't know for sure why a site is unavailable, it did appear that attacks were responsible for most of the year-over-year increase.