Slideshow: Quiz: Separate Cyber Security Fact From Fiction

It's now one of the top problems being addressed by the National Cyber Forensics and Training Alliance (NCFTA), which works with the FBI and industry to share information about cyber attacks, according to NCFTA Executive Director Ron Plesco. "Every year there seems to be a trend and this has been the trend this year," he said.

There has been a "significant increase" in what's known as ACH (automated clearinghouse) fraud over the past few months, much of it targeting small businesses, municipal governments and schools, the FBI said in an alert posted to its Web site.

The criminals can move thousands or even millions of dollars out of their victims' accounts very quickly, using online banking to add new payees to the organization's bank account and then moving the money overnight. Usually the first step is an e-mail to the company's bookkeeper or financial officer that can include malicious attachments designed to look like Microsoft software patches, or simply links to malicious Web sites. The idea is to get the criminal's keylogging software onto a computer with online banking access and then steal login credentials.

Once they have access to the bank account, the hackers set up ACH transfers to money mules -- typically innocent victims who think they're doing payroll processing for international companies -- who then transfer the money overseas via services such as Western Union and Moneygram.

In one case, the criminals even launched a distributed denial-of-service attack against an ACH processor to prevent the bank from recalling transfers before the money mules could move them overseas.

Once the money is out of the country, it is gone for good.

Criminals prefer smaller organizations such as school boards because they tend to work with smaller regional banks that may not have the fraud detection controls in place to stop these fake ACH transfers. These organizations often publish contact information for financial personnel, or even organizational charts posted to their Web sites, making them easy pickings for fraudsters.

According to a report by the FBI's Internet Crime Complaint Center (IC3), banks and financial service providers are often part of the problem. Based on FBI interviews, the IC3 concluded that "in several cases banks did not have proper firewalls installed, nor anti-virus software on their servers or their desktop computers. The lack of defense-in-depth at the smaller institution/service provider level has created a threat to the ACH system."